<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<link:linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <link:roleRef roleURI="http://www.selectabio.com/role/AccruedExpenses" xlink:href="selb-20190331.xsd#AccruedExpenses" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/AccruedExpensesDetails" xlink:href="selb-20190331.xsd#AccruedExpensesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/AccruedExpensesTables" xlink:href="selb-20190331.xsd#AccruedExpensesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/AvailableForSaleMarketableSecurities" xlink:href="selb-20190331.xsd#AvailableForSaleMarketableSecurities" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/AvailableForSaleMarketableSecuritiesScheduleOfFairValuesAndAmortizedCostOfMarketableDebtSecuritiesByContractualMaturityDetails" xlink:href="selb-20190331.xsd#AvailableForSaleMarketableSecuritiesScheduleOfFairValuesAndAmortizedCostOfMarketableDebtSecuritiesByContractualMaturityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/AvailableForSaleMarketableSecuritiesSummaryOfAvailableForSaleMarketableSecuritiesDetails" xlink:href="selb-20190331.xsd#AvailableForSaleMarketableSecuritiesSummaryOfAvailableForSaleMarketableSecuritiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/AvailableForSaleMarketableSecuritiesTables" xlink:href="selb-20190331.xsd#AvailableForSaleMarketableSecuritiesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/CommitmentsAndContingencies" xlink:href="selb-20190331.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/ConsolidatedBalanceSheets" xlink:href="selb-20190331.xsd#ConsolidatedBalanceSheets" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:href="selb-20190331.xsd#ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/ConsolidatedStatementsOfCashFlows" xlink:href="selb-20190331.xsd#ConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xlink:href="selb-20190331.xsd#ConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityDeficitParenthetical" xlink:href="selb-20190331.xsd#ConsolidatedStatementsOfChangesInStockholdersEquityDeficitParenthetical" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveLoss" xlink:href="selb-20190331.xsd#ConsolidatedStatementsOfOperationsAndComprehensiveLoss" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/Debt" xlink:href="selb-20190331.xsd#Debt" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/DebtScheduleOfFutureMinimumPaymentsOnTermLoansDetails" xlink:href="selb-20190331.xsd#DebtScheduleOfFutureMinimumPaymentsOnTermLoansDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/DebtTables" xlink:href="selb-20190331.xsd#DebtTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/DebtTermLoansNarrativeDetails" xlink:href="selb-20190331.xsd#DebtTermLoansNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/DefinedContributionPlan" xlink:href="selb-20190331.xsd#DefinedContributionPlan" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/DefinedContributionPlanDetails" xlink:href="selb-20190331.xsd#DefinedContributionPlanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/DocumentAndEntityInformation" xlink:href="selb-20190331.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/FairValueMeasurements" xlink:href="selb-20190331.xsd#FairValueMeasurements" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/FairValueMeasurementsSummaryOfAssetsMeasuredAtFairValueOnRecurringBasisDetails" xlink:href="selb-20190331.xsd#FairValueMeasurementsSummaryOfAssetsMeasuredAtFairValueOnRecurringBasisDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/FairValueMeasurementsTables" xlink:href="selb-20190331.xsd#FairValueMeasurementsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/IncomeTaxes" xlink:href="selb-20190331.xsd#IncomeTaxes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/IncomeTaxesDetails" xlink:href="selb-20190331.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesComponentsOfLeaseCostsDetails" xlink:href="selb-20190331.xsd#LeasesComponentsOfLeaseCostsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesDetails" xlink:href="selb-20190331.xsd#LeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesLeaseCommitmentsDetails" xlink:href="selb-20190331.xsd#LeasesLeaseCommitmentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesLeaseTermAndDiscountRateDetails" xlink:href="selb-20190331.xsd#LeasesLeaseTermAndDiscountRateDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesNotes" xlink:href="selb-20190331.xsd#LeasesNotes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesOperatingLeaseLiabilityMaturityDetails" xlink:href="selb-20190331.xsd#LeasesOperatingLeaseLiabilityMaturityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesOtherInformationDetails" xlink:href="selb-20190331.xsd#LeasesOtherInformationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/LeasesTables" xlink:href="selb-20190331.xsd#LeasesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NarrativeDetails" xlink:href="selb-20190331.xsd#NarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NatureOfBusinessAndBasisOfPresentation" xlink:href="selb-20190331.xsd#NatureOfBusinessAndBasisOfPresentation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NatureOfBusinessAndBasisOfPresentationLiquidityDetails" xlink:href="selb-20190331.xsd#NatureOfBusinessAndBasisOfPresentationLiquidityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NetLossPerShare" xlink:href="selb-20190331.xsd#NetLossPerShare" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NetLossPerShareComputationOfBasicAndDilutedNetLossPerShareDetails" xlink:href="selb-20190331.xsd#NetLossPerShareComputationOfBasicAndDilutedNetLossPerShareDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NetLossPerShareScheduleOfPotentialCommonSharesIssuableUponConversionOfWarrantsDetails" xlink:href="selb-20190331.xsd#NetLossPerShareScheduleOfPotentialCommonSharesIssuableUponConversionOfWarrantsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NetLossPerShareTables" xlink:href="selb-20190331.xsd#NetLossPerShareTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/NumberOfSharesOfCommonStockPurchasedDetails" xlink:href="selb-20190331.xsd#NumberOfSharesOfCommonStockPurchasedDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsDetails" xlink:href="selb-20190331.xsd#PrepaidExpensesAndOtherCurrentAssetsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsNarrativeDetails" xlink:href="selb-20190331.xsd#PrepaidExpensesAndOtherCurrentAssetsNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsNotes" xlink:href="selb-20190331.xsd#PrepaidExpensesAndOtherCurrentAssetsNotes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsTables" xlink:href="selb-20190331.xsd#PrepaidExpensesAndOtherCurrentAssetsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PropertyAndEquipment" xlink:href="selb-20190331.xsd#PropertyAndEquipment" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PropertyAndEquipmentNarrativeDetails" xlink:href="selb-20190331.xsd#PropertyAndEquipmentNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PropertyAndEquipmentScheduleOfPropertyAndEquipmentDetails" xlink:href="selb-20190331.xsd#PropertyAndEquipmentScheduleOfPropertyAndEquipmentDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/PropertyAndEquipmentTables" xlink:href="selb-20190331.xsd#PropertyAndEquipmentTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RelatedPartyTransactions" xlink:href="selb-20190331.xsd#RelatedPartyTransactions" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RelatedPartyTransactionsTables" xlink:href="selb-20190331.xsd#RelatedPartyTransactionsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RevenueArrangements" xlink:href="selb-20190331.xsd#RevenueArrangements" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RevenueArrangementsNarrativeDetails" xlink:href="selb-20190331.xsd#RevenueArrangementsNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RevenueArrangementsScheduleOfChangesInContractLiabilitiesDetails" xlink:href="selb-20190331.xsd#RevenueArrangementsScheduleOfChangesInContractLiabilitiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RevenueArrangementsTables" xlink:href="selb-20190331.xsd#RevenueArrangementsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/RevenueArrangementsTransactionPriceAllocatedToFuturePerformanceObligationDetails" xlink:href="selb-20190331.xsd#RevenueArrangementsTransactionPriceAllocatedToFuturePerformanceObligationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlans" xlink:href="selb-20190331.xsd#StockIncentivePlans" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansAdditionalInformationNarrativeDetails" xlink:href="selb-20190331.xsd#StockIncentivePlansAdditionalInformationNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansEmployeeStockPurchasePlanNarrativeDetails" xlink:href="selb-20190331.xsd#StockIncentivePlansEmployeeStockPurchasePlanNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansRestrictedStockUnitsDetails" xlink:href="selb-20190331.xsd#StockIncentivePlansRestrictedStockUnitsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansScheduleOfAssumptionsDetails" xlink:href="selb-20190331.xsd#StockIncentivePlansScheduleOfAssumptionsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansScheduleOfStockBasedCompensationExpenseRelatedToStockOptionsAndRestrictedCommonStockDetails" xlink:href="selb-20190331.xsd#StockIncentivePlansScheduleOfStockBasedCompensationExpenseRelatedToStockOptionsAndRestrictedCommonStockDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansSummaryOfActivityUnder2008PlanAnd2016PlanDetails" xlink:href="selb-20190331.xsd#StockIncentivePlansSummaryOfActivityUnder2008PlanAnd2016PlanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockIncentivePlansTables" xlink:href="selb-20190331.xsd#StockIncentivePlansTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockholdersEquity" xlink:href="selb-20190331.xsd#StockholdersEquity" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockholdersEquityNarrativeDetails" xlink:href="selb-20190331.xsd#StockholdersEquityNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockholdersEquityScheduleOfAuthorizedSharesOfCommonStockForFutureIssuanceDetails" xlink:href="selb-20190331.xsd#StockholdersEquityScheduleOfAuthorizedSharesOfCommonStockForFutureIssuanceDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/StockholdersEquityTables" xlink:href="selb-20190331.xsd#StockholdersEquityTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesCashEquivalentsShortTermInvestmentsAndRestrictedCashDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesCashEquivalentsShortTermInvestmentsAndRestrictedCashDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesComponentsOfAccumulatedOtherComprehensiveIncomeLossNetOfTaxDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesComponentsOfAccumulatedOtherComprehensiveIncomeLossNetOfTaxDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesConcentrationsOfCreditRiskAndOffBalanceSheetRiskDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesConcentrationsOfCreditRiskAndOffBalanceSheetRiskDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesImpairmentOfLonglivedAssetsDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesImpairmentOfLonglivedAssetsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesPropertyAndEquipmentDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesPropertyAndEquipmentDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesScheduleOfReconciliationOfCashCashEquivalentsAndRestrictedCashDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesScheduleOfReconciliationOfCashCashEquivalentsAndRestrictedCashDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesSegmentInformationDetails" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesSegmentInformationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:href="selb-20190331.xsd#SummaryOfSignificantAccountingPoliciesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/TechnologyLicenseAgreements" xlink:href="selb-20190331.xsd#TechnologyLicenseAgreements" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/TechnologyLicenseAgreementsMassachusettsEyeAndEarInfirmaryAndSchepensEyeResearchInstituteIncNarrativeDetails" xlink:href="selb-20190331.xsd#TechnologyLicenseAgreementsMassachusettsEyeAndEarInfirmaryAndSchepensEyeResearchInstituteIncNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/TechnologyLicenseAgreementsMitNarrativeDetails" xlink:href="selb-20190331.xsd#TechnologyLicenseAgreementsMitNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.selectabio.com/role/TechnologyLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails" xlink:href="selb-20190331.xsd#TechnologyLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AccruedExpenses" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AccruedExpensesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AccruedExpensesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AvailableForSaleMarketableSecurities" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AvailableForSaleMarketableSecuritiesScheduleOfFairValuesAndAmortizedCostOfMarketableDebtSecuritiesByContractualMaturityDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AvailableForSaleMarketableSecuritiesSummaryOfAvailableForSaleMarketableSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_9CFFB058BE2DF524A27197E921363F17" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_5A5DFF429C7488D9C2CB97E92135227B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_9CFFB058BE2DF524A27197E921363F17" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_5A5DFF429C7488D9C2CB97E92135227B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="loc_us-gaap_FinancialInstrumentAxis_49A0EC87C1282DA007D197E9213568B4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_5A5DFF429C7488D9C2CB97E92135227B" xlink:to="loc_us-gaap_FinancialInstrumentAxis_49A0EC87C1282DA007D197E9213568B4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6B3EB94CB0F9AB7B14EF97E921365D45_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_49A0EC87C1282DA007D197E9213568B4" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6B3EB94CB0F9AB7B14EF97E921365D45_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6B3EB94CB0F9AB7B14EF97E921365D45" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_49A0EC87C1282DA007D197E9213568B4" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6B3EB94CB0F9AB7B14EF97E921365D45" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_D4AB687DD9E0B53B948D97E92136AE49" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6B3EB94CB0F9AB7B14EF97E921365D45" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_D4AB687DD9E0B53B948D97E92136AE49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CorporateBondSecuritiesMember" xlink:label="loc_us-gaap_CorporateBondSecuritiesMember_6394B4CE3D0118C7827B97E921368CD2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_6B3EB94CB0F9AB7B14EF97E921365D45" xlink:to="loc_us-gaap_CorporateBondSecuritiesMember_6394B4CE3D0118C7827B97E921368CD2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_7F2DADEE106E9E78832C97E92137AF32" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_9CFFB058BE2DF524A27197E921363F17" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_7F2DADEE106E9E78832C97E92137AF32" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_AF40727B8027B2F3C02297E921374C36" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_9CFFB058BE2DF524A27197E921363F17" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_AF40727B8027B2F3C02297E921374C36" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_32672A0C7C39DCECAC7E97E92137F3F0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_9CFFB058BE2DF524A27197E921363F17" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_32672A0C7C39DCECAC7E97E92137F3F0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_76BF27D760F46F83AFA097E92137480D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_9CFFB058BE2DF524A27197E921363F17" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_76BF27D760F46F83AFA097E92137480D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/AvailableForSaleMarketableSecuritiesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/ConsolidatedBalanceSheets" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/ConsolidatedStatementsOfCashFlows" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityDeficit" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_BACA7340BE4E9953E73197E921C4AE8D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_52E743291FA0EC488CCF97E921C2264A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_BACA7340BE4E9953E73197E921C4AE8D" xlink:to="loc_us-gaap_StatementTable_52E743291FA0EC488CCF97E921C2264A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_B135E143564951BB3E7D97E921C38AE9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_52E743291FA0EC488CCF97E921C2264A" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_B135E143564951BB3E7D97E921C38AE9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_B135E143564951BB3E7D97E921C38AE9" xlink:to="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_B135E143564951BB3E7D97E921C38AE9" xlink:to="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_DDC1007971B3D99F0C4697E921C32A0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6" xlink:to="loc_us-gaap_CommonStockMember_DDC1007971B3D99F0C4697E921C32A0F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_1AAA096D6BC73A9CF78897E921C3B01B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_1AAA096D6BC73A9CF78897E921C3B01B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_014516052A3EF30EDF6C97E921C4D0AF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6" xlink:to="loc_us-gaap_RetainedEarningsMember_014516052A3EF30EDF6C97E921C4D0AF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_535D1B6780FC8564794397E921C4710E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7B08639F7A4A11040DA997E921C3F7F6" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_535D1B6780FC8564794397E921C4710E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_BACA7340BE4E9953E73197E921C4AE8D" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_6B02BFE5081E7222B2AF97E921C4592C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_6B02BFE5081E7222B2AF97E921C4592C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_412054CA73A4E250A4DB97E921C5D8AB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockholdersEquity_412054CA73A4E250A4DB97E921C5D8AB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_275F044562FB145C3BDA97E921C5750C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_275F044562FB145C3BDA97E921C5750C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_E554E2AE800F473F7C5197E921C538AB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_E554E2AE800F473F7C5197E921C538AB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_11D6C497EB40A14C39D797E921C5B812" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_11D6C497EB40A14C39D797E921C5B812" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_99B179C03A1DB6B4DDCB97E921C56298" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_99B179C03A1DB6B4DDCB97E921C56298" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_3E7770D5761A39AAC82F97E921C6B1E0" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_3E7770D5761A39AAC82F97E921C6B1E0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_7345DF40794498CE5F1F97E921C6F817" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_7345DF40794498CE5F1F97E921C6F817" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_DA00095C6796B344A04797E921C6C9B3" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_DA00095C6796B344A04797E921C6C9B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_DE922BC053B71E13AF1697E921C65125" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_DE922BC053B71E13AF1697E921C65125" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_6A8E91070E4E0DB7E0FB97E921C6FA58" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_6A8E91070E4E0DB7E0FB97E921C6FA58" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_E2F4A04CCD940A5DAC7D97E921C64961" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_E3A49C3B7038CB87630F97E921C40D42" xlink:to="loc_us-gaap_NetIncomeLoss_E2F4A04CCD940A5DAC7D97E921C64961" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityDeficitParenthetical" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveLoss" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/Debt" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/DebtScheduleOfFutureMinimumPaymentsOnTermLoansDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_288D358027970EEE45499414AE5F0F38" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_11F6E9B8CC075F26B68A9414AE5DA4CD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_288D358027970EEE45499414AE5F0F38" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_11F6E9B8CC075F26B68A9414AE5DA4CD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_C73E2C9EA1D1191EDCC79414AE5ECE29" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_11F6E9B8CC075F26B68A9414AE5DA4CD" xlink:to="loc_us-gaap_CreditFacilityAxis_C73E2C9EA1D1191EDCC79414AE5ECE29" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_86FF1FCFF02650D51A889414AE5EBC0F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_C73E2C9EA1D1191EDCC79414AE5ECE29" xlink:to="loc_us-gaap_CreditFacilityDomain_86FF1FCFF02650D51A889414AE5EBC0F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_86FF1FCFF02650D51A889414AE5EBC0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_C73E2C9EA1D1191EDCC79414AE5ECE29" xlink:to="loc_us-gaap_CreditFacilityDomain_86FF1FCFF02650D51A889414AE5EBC0F" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_A2017TermLoansMember" xlink:label="loc_selb_A2017TermLoansMember_9858452B77A02B722D9C9414AE5ED62C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_86FF1FCFF02650D51A889414AE5EBC0F" xlink:to="loc_selb_A2017TermLoansMember_9858452B77A02B722D9C9414AE5ED62C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtByMaturityAbstract" xlink:label="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_288D358027970EEE45499414AE5F0F38" xlink:to="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_2592D2E540305ADAA52F942628AA3F86" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalRemainderOfFiscalYear_2592D2E540305ADAA52F942628AA3F86" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_2D1A3E2E92ED6C8C286C9414AE5F53F9" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo_2D1A3E2E92ED6C8C286C9414AE5F53F9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_2AEFD714E49329B793149414AE5FD692" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree_2AEFD714E49329B793149414AE5FD692" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour" xlink:label="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour_31BDD84FB6DB63F9AA489414AE6032FC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour_31BDD84FB6DB63F9AA489414AE6032FC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebt" xlink:label="loc_us-gaap_LongTermDebt_4F557637246F44991AAF9414AE609D34" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebt_4F557637246F44991AAF9414AE609D34" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DebtInstrumentInterestPortion" xlink:label="loc_selb_DebtInstrumentInterestPortion_5EAFC80D551C47A1556F9414AE6006B2" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_selb_DebtInstrumentInterestPortion_5EAFC80D551C47A1556F9414AE6006B2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_ADA7C897B1BC22BBF1F09414AE60263F" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_ADA7C897B1BC22BBF1F09414AE60263F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtCurrent" xlink:label="loc_us-gaap_LongTermDebtCurrent_57F478E577F2CDBC89939414AE60E25D" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebtCurrent_57F478E577F2CDBC89939414AE60E25D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LongTermDebtNoncurrent" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_8949C3423CC37164CD679414AE602620" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtByMaturityAbstract_FEE217BDC521630A22079414AE5F97D0" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_8949C3423CC37164CD679414AE602620" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/DebtTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/DebtTermLoansNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityLineItems" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityTable" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_1B4360BC573EA344B28169D2ED55EB1D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_1B4360BC573EA344B28169D2ED55EB1D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_D6F3FC06FE787730287F69D2ED55D3A7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_1B4360BC573EA344B28169D2ED55EB1D" xlink:to="loc_us-gaap_CreditFacilityAxis_D6F3FC06FE787730287F69D2ED55D3A7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_57363D1456E3FE1D016269D2ED55411E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_D6F3FC06FE787730287F69D2ED55D3A7" xlink:to="loc_us-gaap_CreditFacilityDomain_57363D1456E3FE1D016269D2ED55411E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_57363D1456E3FE1D016269D2ED55411E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_D6F3FC06FE787730287F69D2ED55D3A7" xlink:to="loc_us-gaap_CreditFacilityDomain_57363D1456E3FE1D016269D2ED55411E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_A2017TermLoansMember" xlink:label="loc_selb_A2017TermLoansMember_73E88AD128A2896C00BF69D2ED56E208" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_57363D1456E3FE1D016269D2ED55411E" xlink:to="loc_selb_A2017TermLoansMember_73E88AD128A2896C00BF69D2ED56E208" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_TermLoanMember" xlink:label="loc_selb_TermLoanMember_16CC8C28EC18DA3185E269D2ED567E20" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_57363D1456E3FE1D016269D2ED55411E" xlink:to="loc_selb_TermLoanMember_16CC8C28EC18DA3185E269D2ED567E20" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DebtCovenantPeriodAxis" xlink:label="loc_selb_DebtCovenantPeriodAxis_B397D28E52187D84F35C69D2ED5642C0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_1B4360BC573EA344B28169D2ED55EB1D" xlink:to="loc_selb_DebtCovenantPeriodAxis_B397D28E52187D84F35C69D2ED5642C0" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DebtCovenantPeriodDomain" xlink:label="loc_selb_DebtCovenantPeriodDomain_9A10609E6F02C9314CD369D2ED57B5B3_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_selb_DebtCovenantPeriodAxis_B397D28E52187D84F35C69D2ED5642C0" xlink:to="loc_selb_DebtCovenantPeriodDomain_9A10609E6F02C9314CD369D2ED57B5B3_default" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DebtCovenantPeriodDomain" xlink:label="loc_selb_DebtCovenantPeriodDomain_9A10609E6F02C9314CD369D2ED57B5B3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_selb_DebtCovenantPeriodAxis_B397D28E52187D84F35C69D2ED5642C0" xlink:to="loc_selb_DebtCovenantPeriodDomain_9A10609E6F02C9314CD369D2ED57B5B3" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_TermLoanPrepaidAfterFirstAnniversaryButBeforeSecondAnniversaryMember" xlink:label="loc_selb_TermLoanPrepaidAfterFirstAnniversaryButBeforeSecondAnniversaryMember_11CAE16D2B8D8F662DDE69D2ED57A55C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_DebtCovenantPeriodDomain_9A10609E6F02C9314CD369D2ED57B5B3" xlink:to="loc_selb_TermLoanPrepaidAfterFirstAnniversaryButBeforeSecondAnniversaryMember_11CAE16D2B8D8F662DDE69D2ED57A55C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_TermLoanPrepaidAfterSecondAnniversaryMember" xlink:label="loc_selb_TermLoanPrepaidAfterSecondAnniversaryMember_57B752ED117DA05784B169D2ED5736F8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_DebtCovenantPeriodDomain_9A10609E6F02C9314CD369D2ED57B5B3" xlink:to="loc_selb_TermLoanPrepaidAfterSecondAnniversaryMember_57B752ED117DA05784B169D2ED5736F8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VariableRateAxis" xlink:label="loc_us-gaap_VariableRateAxis_05FFA4BB84B620027D4D69D2ED606E87" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_1B4360BC573EA344B28169D2ED55EB1D" xlink:to="loc_us-gaap_VariableRateAxis_05FFA4BB84B620027D4D69D2ED606E87" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_6639A7EDD499798976C869D2ED60FF9A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_05FFA4BB84B620027D4D69D2ED606E87" xlink:to="loc_us-gaap_VariableRateDomain_6639A7EDD499798976C869D2ED60FF9A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VariableRateDomain" xlink:label="loc_us-gaap_VariableRateDomain_6639A7EDD499798976C869D2ED60FF9A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_05FFA4BB84B620027D4D69D2ED606E87" xlink:to="loc_us-gaap_VariableRateDomain_6639A7EDD499798976C869D2ED60FF9A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_BaseRateMember" xlink:label="loc_us-gaap_BaseRateMember_9B3001CD5D0B1F4B453D69D2ED6081CD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_6639A7EDD499798976C869D2ED60FF9A" xlink:to="loc_us-gaap_BaseRateMember_9B3001CD5D0B1F4B453D69D2ED6081CD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCredit" xlink:label="loc_us-gaap_LineOfCredit_6EB54080FE2F902CD0D569D2ED63BEC7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_LineOfCredit_6EB54080FE2F902CD0D569D2ED63BEC7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RepaymentsOfLinesOfCredit" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_864C5FDC84CA27F8E37E69D2ED6354FF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_864C5FDC84CA27F8E37E69D2ED6354FF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_C074BCA7119DFE7BC2BE69D2ED63170D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_C074BCA7119DFE7BC2BE69D2ED63170D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet" xlink:label="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_E79091CD7074DAD341CF69D2ED632E91" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_E79091CD7074DAD341CF69D2ED632E91" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EarlyExtinguishmentOfDebtPrepaymentFeePercentage" xlink:label="loc_selb_EarlyExtinguishmentOfDebtPrepaymentFeePercentage_A90573F1AB2A0AB4AAE969D2ED619221" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_selb_EarlyExtinguishmentOfDebtPrepaymentFeePercentage_A90573F1AB2A0AB4AAE969D2ED619221" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LineOfCreditFacilityInterestRateDuringPeriod" xlink:label="loc_us-gaap_LineOfCreditFacilityInterestRateDuringPeriod_226E8A4E137797F3ED4769D2ED61D448" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_LineOfCreditFacilityInterestRateDuringPeriod_226E8A4E137797F3ED4769D2ED61D448" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_FinalPaymentFeePercentage" xlink:label="loc_selb_FinalPaymentFeePercentage_73A5B1E5329E2A58A6CA69D2ED61F7AE" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_selb_FinalPaymentFeePercentage_73A5B1E5329E2A58A6CA69D2ED61F7AE" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DebtInstrumentEventOfDefaultMinimumAssetsTrigger" xlink:label="loc_selb_DebtInstrumentEventOfDefaultMinimumAssetsTrigger_1D61AA840A76395227E569D2ED637840" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_selb_DebtInstrumentEventOfDefaultMinimumAssetsTrigger_1D61AA840A76395227E569D2ED637840" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DebtInstrumentEventOfDefaultMinimumAmountInExcessOfIndebtednessTrigger" xlink:label="loc_selb_DebtInstrumentEventOfDefaultMinimumAmountInExcessOfIndebtednessTrigger_7EB0BFEB0F6F2DF0FA5969D2ED6434D9" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_selb_DebtInstrumentEventOfDefaultMinimumAmountInExcessOfIndebtednessTrigger_7EB0BFEB0F6F2DF0FA5969D2ED6434D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:label="loc_us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid_7008CC45C01A920F58BF69D2ED62ADA0" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid_7008CC45C01A920F58BF69D2ED62ADA0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InterestExpense" xlink:label="loc_us-gaap_InterestExpense_E49E673BC6027E8D1B9869D2ED649CF1" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_InterestExpense_E49E673BC6027E8D1B9869D2ED649CF1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_9CAAF2F4790724DA106669D2ED64A9A9" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_BDA123C2A6A58FC41DBF69D2ED603BAA" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_9CAAF2F4790724DA106669D2ED64A9A9" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/DefinedContributionPlan" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/DefinedContributionPlanDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/DocumentAndEntityInformation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/FairValueMeasurements" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/FairValueMeasurementsSummaryOfAssetsMeasuredAtFairValueOnRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_3DE2CEFE03FF8A9C5B3B93F56053480E" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_F4BADF472A85B1FE6F4C93F5604F9077" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_3DE2CEFE03FF8A9C5B3B93F56053480E" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_F4BADF472A85B1FE6F4C93F5604F9077" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FA8BE5D8071CDAD7032493F5604F9E6A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_F4BADF472A85B1FE6F4C93F5604F9077" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FA8BE5D8071CDAD7032493F5604F9E6A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_31818655D971766C122093F5605019A5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FA8BE5D8071CDAD7032493F5604F9E6A" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_31818655D971766C122093F5605019A5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_31818655D971766C122093F5605019A5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_FA8BE5D8071CDAD7032493F5604F9E6A" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_31818655D971766C122093F5605019A5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_B4DC947B46EDB28984A993F56050EF95" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_31818655D971766C122093F5605019A5" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_B4DC947B46EDB28984A993F56050EF95" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_AE6B48366E3711952B7293F56050B031" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_F4BADF472A85B1FE6F4C93F5604F9077" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_AE6B48366E3711952B7293F56050B031" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_BB340CAAFB3CE5A4164E93F560508E87_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_AE6B48366E3711952B7293F56050B031" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_BB340CAAFB3CE5A4164E93F560508E87_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_BB340CAAFB3CE5A4164E93F560508E87" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_AE6B48366E3711952B7293F56050B031" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_BB340CAAFB3CE5A4164E93F560508E87" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="loc_us-gaap_MoneyMarketFundsMember_EBAA7DF1FD33DA45AC6E93F5605092E9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_BB340CAAFB3CE5A4164E93F560508E87" xlink:to="loc_us-gaap_MoneyMarketFundsMember_EBAA7DF1FD33DA45AC6E93F5605092E9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="loc_us-gaap_FinancialInstrumentAxis_4230EF4890604E0C802593F56051C9B7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_F4BADF472A85B1FE6F4C93F5604F9077" xlink:to="loc_us-gaap_FinancialInstrumentAxis_4230EF4890604E0C802593F56051C9B7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_A7CAE3F00B2CD77F803793F560519B63_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_4230EF4890604E0C802593F56051C9B7" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_A7CAE3F00B2CD77F803793F560519B63_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_A7CAE3F00B2CD77F803793F560519B63" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_4230EF4890604E0C802593F56051C9B7" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_A7CAE3F00B2CD77F803793F560519B63" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_158727B8805C7A5CD3F193F560516B23" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_A7CAE3F00B2CD77F803793F560519B63" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_158727B8805C7A5CD3F193F560516B23" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_CorporateBondsAndCommercialPaperMember" xlink:label="loc_selb_CorporateBondsAndCommercialPaperMember_13879AE25923A1DA5B5593F560512C10" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_A7CAE3F00B2CD77F803793F560519B63" xlink:to="loc_selb_CorporateBondsAndCommercialPaperMember_13879AE25923A1DA5B5593F560512C10" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CorporateBondSecuritiesMember" xlink:label="loc_us-gaap_CorporateBondSecuritiesMember_6EB7EDFB5F4CEF745E0D93F56052B18C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_CorporateBondsAndCommercialPaperMember_13879AE25923A1DA5B5593F560512C10" xlink:to="loc_us-gaap_CorporateBondSecuritiesMember_6EB7EDFB5F4CEF745E0D93F56052B18C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommercialPaperMember" xlink:label="loc_us-gaap_CommercialPaperMember_3B4F3ED3345F7BF6E70093F560523C1A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_CorporateBondsAndCommercialPaperMember_13879AE25923A1DA5B5593F560512C10" xlink:to="loc_us-gaap_CommercialPaperMember_3B4F3ED3345F7BF6E70093F560523C1A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_470C63A4859A6499E77093F560521DAB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_F4BADF472A85B1FE6F4C93F5604F9077" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_470C63A4859A6499E77093F560521DAB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_470C63A4859A6499E77093F560521DAB" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_470C63A4859A6499E77093F560521DAB" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_D1A7CD4B88570A1B614D93F560524FE1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_D1A7CD4B88570A1B614D93F560524FE1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_3C904CFF277632D8083393F56053C3A2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_3C904CFF277632D8083393F56053C3A2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_6B19657FD224FDFC1FC593F56053587C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_348AD0D6E688E63BFF6593F56052717C" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_6B19657FD224FDFC1FC593F56053587C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_3FC7819E1ECF3AEE851C93F560533AFD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_3DE2CEFE03FF8A9C5B3B93F56053480E" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_3FC7819E1ECF3AEE851C93F560533AFD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_InvestmentsFairValueDisclosure" xlink:label="loc_us-gaap_InvestmentsFairValueDisclosure_F2545CE2C6A9A2F88D0893F56053FE81" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_3DE2CEFE03FF8A9C5B3B93F56053480E" xlink:to="loc_us-gaap_InvestmentsFairValueDisclosure_F2545CE2C6A9A2F88D0893F56053FE81" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_InvestmentsMaturityPeriod" xlink:label="loc_selb_InvestmentsMaturityPeriod_F9EB2B3ED4CB62D8021993F5605414A1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_3DE2CEFE03FF8A9C5B3B93F56053480E" xlink:to="loc_selb_InvestmentsMaturityPeriod_F9EB2B3ED4CB62D8021993F5605414A1" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/FairValueMeasurementsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/IncomeTaxes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/IncomeTaxesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxHolidayLineItems" xlink:label="loc_us-gaap_IncomeTaxHolidayLineItems_BC020AA04D4EA45E60CF94115CCED163" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxHolidayTable" xlink:label="loc_us-gaap_IncomeTaxHolidayTable_E86F0CC111644E8305C194115CCC242F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxHolidayLineItems_BC020AA04D4EA45E60CF94115CCED163" xlink:to="loc_us-gaap_IncomeTaxHolidayTable_E86F0CC111644E8305C194115CCC242F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_0D88F625282DBEC9668094115CCD5E27" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxHolidayTable_E86F0CC111644E8305C194115CCC242F" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_0D88F625282DBEC9668094115CCD5E27" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_CFAA4011EF9F7DAA589D94115CCD7F6C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_0D88F625282DBEC9668094115CCD5E27" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_CFAA4011EF9F7DAA589D94115CCD7F6C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_CFAA4011EF9F7DAA589D94115CCD7F6C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_0D88F625282DBEC9668094115CCD5E27" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_CFAA4011EF9F7DAA589D94115CCD7F6C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DomesticAndStateTaxAuthorityMember" xlink:label="loc_selb_DomesticAndStateTaxAuthorityMember_EFB9A3DE14D3B3D4179194115CCEB8A6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_CFAA4011EF9F7DAA589D94115CCD7F6C" xlink:to="loc_selb_DomesticAndStateTaxAuthorityMember_EFB9A3DE14D3B3D4179194115CCEB8A6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_40D68603E594B602B27294115CCEE686" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxHolidayLineItems_BC020AA04D4EA45E60CF94115CCED163" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration_40D68603E594B602B27294115CCEE686" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UnrecognizedTaxBenefits" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_5E61E0237A00B0AED57194115CCEF7BA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxHolidayLineItems_BC020AA04D4EA45E60CF94115CCED163" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_5E61E0237A00B0AED57194115CCEF7BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_B58BB71053EE1AB8A3A2941469C9B56D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxHolidayLineItems_BC020AA04D4EA45E60CF94115CCED163" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_B58BB71053EE1AB8A3A2941469C9B56D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesComponentsOfLeaseCostsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeasedAssetsLineItems" xlink:label="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfOperatingLeasedAssetsTable" xlink:label="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_16EDDF191143BBE27F249882D3CADD42" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_16EDDF191143BBE27F249882D3CADD42" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_F6F5A0657C465A43B33A9882D3CAFD8F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_16EDDF191143BBE27F249882D3CADD42" xlink:to="loc_us-gaap_TypeOfAdoptionMember_F6F5A0657C465A43B33A9882D3CAFD8F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_F6F5A0657C465A43B33A9882D3CAFD8F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_16EDDF191143BBE27F249882D3CADD42" xlink:to="loc_us-gaap_TypeOfAdoptionMember_F6F5A0657C465A43B33A9882D3CAFD8F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201602Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201602Member_D55CC26DFE8168FA563F9882D3CB9103" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_F6F5A0657C465A43B33A9882D3CAFD8F" xlink:to="loc_us-gaap_AccountingStandardsUpdate201602Member_D55CC26DFE8168FA563F9882D3CB9103" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_96E0AB51BFB9EE19F0869882D3CBCC96" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:to="loc_srt_RangeAxis_96E0AB51BFB9EE19F0869882D3CBCC96" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_BE3000D29280B4AB65299882D3CBC5D8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_96E0AB51BFB9EE19F0869882D3CBCC96" xlink:to="loc_srt_RangeMember_BE3000D29280B4AB65299882D3CBC5D8_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_BE3000D29280B4AB65299882D3CBC5D8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_96E0AB51BFB9EE19F0869882D3CBCC96" xlink:to="loc_srt_RangeMember_BE3000D29280B4AB65299882D3CBC5D8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_84FFF1B35BA0EC57389A9882D3CB5216" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_BE3000D29280B4AB65299882D3CBC5D8" xlink:to="loc_srt_MinimumMember_84FFF1B35BA0EC57389A9882D3CB5216" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_F54EBF6E9F77A2843B469882D3CC16D4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_BE3000D29280B4AB65299882D3CBC5D8" xlink:to="loc_srt_MaximumMember_F54EBF6E9F77A2843B469882D3CC16D4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseArrangementTypeAxis" xlink:label="loc_us-gaap_LeaseArrangementTypeAxis_39817797FD292C25BF459882D3CCB071" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:to="loc_us-gaap_LeaseArrangementTypeAxis_39817797FD292C25BF459882D3CCB071" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="loc_us-gaap_LeaseArrangementTypeDomain_607235FD1B3BB03820D99882D3CC84C8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseArrangementTypeAxis_39817797FD292C25BF459882D3CCB071" xlink:to="loc_us-gaap_LeaseArrangementTypeDomain_607235FD1B3BB03820D99882D3CC84C8_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="loc_us-gaap_LeaseArrangementTypeDomain_607235FD1B3BB03820D99882D3CC84C8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseArrangementTypeAxis_39817797FD292C25BF459882D3CCB071" xlink:to="loc_us-gaap_LeaseArrangementTypeDomain_607235FD1B3BB03820D99882D3CC84C8" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LaboratoryAndOfficeSpaceMember" xlink:label="loc_selb_LaboratoryAndOfficeSpaceMember_8622A27DF860DC84BE899882D3CC4372" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseArrangementTypeDomain_607235FD1B3BB03820D99882D3CC84C8" xlink:to="loc_selb_LaboratoryAndOfficeSpaceMember_8622A27DF860DC84BE899882D3CC4372" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_461234EF8B868F73CFA19882D3CD08E6" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:to="loc_us-gaap_CreditFacilityAxis_461234EF8B868F73CFA19882D3CD08E6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_F06F569C5581BA044F5E9882D3CD6199_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_461234EF8B868F73CFA19882D3CD08E6" xlink:to="loc_us-gaap_CreditFacilityDomain_F06F569C5581BA044F5E9882D3CD6199_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_F06F569C5581BA044F5E9882D3CD6199" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_461234EF8B868F73CFA19882D3CD08E6" xlink:to="loc_us-gaap_CreditFacilityDomain_F06F569C5581BA044F5E9882D3CD6199" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LetterOfCreditMember" xlink:label="loc_us-gaap_LetterOfCreditMember_67FCDF5ED12E05D4FE439882D3CD10E9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_F06F569C5581BA044F5E9882D3CD6199" xlink:to="loc_us-gaap_LetterOfCreditMember_67FCDF5ED12E05D4FE439882D3CD10E9" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LeaseArrangementAxis" xlink:label="loc_selb_LeaseArrangementAxis_6F66793F784FD23FABA49882D3CDD474" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfOperatingLeasedAssetsTable_652ED91F2EB6CA9106A99882D3C9B381" xlink:to="loc_selb_LeaseArrangementAxis_6F66793F784FD23FABA49882D3CDD474" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LeaseArrangementDomain" xlink:label="loc_selb_LeaseArrangementDomain_185D89E1ABE14F773F059882D3CE9B24_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_selb_LeaseArrangementAxis_6F66793F784FD23FABA49882D3CDD474" xlink:to="loc_selb_LeaseArrangementDomain_185D89E1ABE14F773F059882D3CE9B24_default" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LeaseArrangementDomain" xlink:label="loc_selb_LeaseArrangementDomain_185D89E1ABE14F773F059882D3CE9B24" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_selb_LeaseArrangementAxis_6F66793F784FD23FABA49882D3CDD474" xlink:to="loc_selb_LeaseArrangementDomain_185D89E1ABE14F773F059882D3CE9B24" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_A75NorthBeaconWatertownMAMember" xlink:label="loc_selb_A75NorthBeaconWatertownMAMember_48A9D04E4C165C81B5F59882D3CEA823" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_LeaseArrangementDomain_185D89E1ABE14F773F059882D3CE9B24" xlink:to="loc_selb_A75NorthBeaconWatertownMAMember_48A9D04E4C165C81B5F59882D3CEA823" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_DE552DF8C16F5841B6449882D3CED644" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_DE552DF8C16F5841B6449882D3CED644" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OperatingLeaseLiability" xlink:label="loc_us-gaap_OperatingLeaseLiability_9F4D81C7ADB2F9FDE6529882D3CFEC2D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_OperatingLeaseLiability_9F4D81C7ADB2F9FDE6529882D3CFEC2D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DeferredRentCredit" xlink:label="loc_us-gaap_DeferredRentCredit_9AC6160DD4B713AB45E59882D3CF12E5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_DeferredRentCredit_9AC6160DD4B713AB45E59882D3CF12E5" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_OperatingLeaseRemainingLeaseTerm" xlink:label="loc_selb_OperatingLeaseRemainingLeaseTerm_669854E21179EF2FFD8F9882D3CF9FD4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_selb_OperatingLeaseRemainingLeaseTerm_669854E21179EF2FFD8F9882D3CF9FD4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LesseeOperatingLeaseDiscountRate" xlink:label="loc_us-gaap_LesseeOperatingLeaseDiscountRate_0C3534FBF9B30BF61D009882D3CF0CEE" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_LesseeOperatingLeaseDiscountRate_0C3534FBF9B30BF61D009882D3CF0CEE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LesseeOperatingLeaseTermOfContract" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_8F0A20FD0BDA922319459882D3CF055D" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_8F0A20FD0BDA922319459882D3CF055D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_C23D274E31D2DB90117A9882D3CF12E0" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_C23D274E31D2DB90117A9882D3CF12E0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_983BE9946A3A2FE861A89882D3D0BF76" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_983BE9946A3A2FE861A89882D3D0BF76" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedCash" xlink:label="loc_us-gaap_RestrictedCash_49796D20B2755746E5B19882D3D06A9F" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_RestrictedCash_49796D20B2755746E5B19882D3D06A9F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AreaOfRealEstateProperty" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_C1D0B7C027ED7E6B74719882D3D02AB2" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_C1D0B7C027ED7E6B74719882D3D02AB2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseAndRentalExpense" xlink:label="loc_us-gaap_LeaseAndRentalExpense_CDD574F29B6466D673389882D3D06358" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLeasedAssetsLineItems_AC5A1ED780C4CC785D159882D3CE69C8" xlink:to="loc_us-gaap_LeaseAndRentalExpense_CDD574F29B6466D673389882D3D06358" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesLeaseCommitmentsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesLeaseTermAndDiscountRateDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesNotes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesOperatingLeaseLiabilityMaturityDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesOtherInformationDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/LeasesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_AC0F8BA2E71BA0A7EE6294C4CCFE276A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:to="loc_us-gaap_AwardTypeAxis_AC0F8BA2E71BA0A7EE6294C4CCFE276A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_585F0857AF842A5FF6FE94C4CCFEB006_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_AC0F8BA2E71BA0A7EE6294C4CCFE276A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_585F0857AF842A5FF6FE94C4CCFEB006_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_585F0857AF842A5FF6FE94C4CCFEB006" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_AC0F8BA2E71BA0A7EE6294C4CCFE276A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_585F0857AF842A5FF6FE94C4CCFEB006" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NonEmployeeStockOptionMember" xlink:label="loc_selb_NonEmployeeStockOptionMember_5335CA1076E345AA1C5894C4CCFEA5A3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_585F0857AF842A5FF6FE94C4CCFEB006" xlink:to="loc_selb_NonEmployeeStockOptionMember_5335CA1076E345AA1C5894C4CCFEA5A3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_AF221DE3B9A07DBAEA1194C4CCFE012C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_AF221DE3B9A07DBAEA1194C4CCFE012C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_415908529EF4A469291E94C4CCFEAE1E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_AF221DE3B9A07DBAEA1194C4CCFE012C" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_415908529EF4A469291E94C4CCFEAE1E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_415908529EF4A469291E94C4CCFEAE1E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_AF221DE3B9A07DBAEA1194C4CCFE012C" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_415908529EF4A469291E94C4CCFEAE1E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="loc_us-gaap_OverAllotmentOptionMember_C5DCFDD1A977E529E369952AF15F1695" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_415908529EF4A469291E94C4CCFEAE1E" xlink:to="loc_us-gaap_OverAllotmentOptionMember_C5DCFDD1A977E529E369952AF15F1695" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_CA8FB09DC6594BDDD96D94C4CD028DF8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:to="loc_us-gaap_TitleOfIndividualAxis_CA8FB09DC6594BDDD96D94C4CD028DF8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1C11A81DBC8964E8616094C4CD0297BB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_CA8FB09DC6594BDDD96D94C4CD028DF8" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1C11A81DBC8964E8616094C4CD0297BB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1C11A81DBC8964E8616094C4CD0297BB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_CA8FB09DC6594BDDD96D94C4CD028DF8" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1C11A81DBC8964E8616094C4CD0297BB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DirectorMember" xlink:label="loc_us-gaap_DirectorMember_E51906700B5529C1F83394C4CD02AF7E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1C11A81DBC8964E8616094C4CD0297BB" xlink:to="loc_us-gaap_DirectorMember_E51906700B5529C1F83394C4CD02AF7E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_FA5B0BC2466688BD6F4D94C4CD033D6B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_FA5B0BC2466688BD6F4D94C4CD033D6B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_DBC7747C7E61175F786C94C4CD03C1CB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_FA5B0BC2466688BD6F4D94C4CD033D6B" xlink:to="loc_us-gaap_RelatedPartyDomain_DBC7747C7E61175F786C94C4CD03C1CB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_DBC7747C7E61175F786C94C4CD03C1CB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_FA5B0BC2466688BD6F4D94C4CD033D6B" xlink:to="loc_us-gaap_RelatedPartyDomain_DBC7747C7E61175F786C94C4CD03C1CB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DirectorMember" xlink:label="loc_us-gaap_DirectorMember_D2CB5E2F98142AD7AE75952DD99D26E8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_DBC7747C7E61175F786C94C4CD03C1CB" xlink:to="loc_us-gaap_DirectorMember_D2CB5E2F98142AD7AE75952DD99D26E8" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_FounderMember" xlink:label="loc_selb_FounderMember_E437A54189DDD3AD5C0094C4CD031B12" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_DBC7747C7E61175F786C94C4CD03C1CB" xlink:to="loc_selb_FounderMember_E437A54189DDD3AD5C0094C4CD031B12" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_CF780182E603FD736A1394C4CD032433" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_E0FCD56F2A3B7D5A672394C4CCFE34B1" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_CF780182E603FD736A1394C4CD032433" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_3E82AD85A1D6273303C894C4CD03ED6B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_CF780182E603FD736A1394C4CD032433" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_3E82AD85A1D6273303C894C4CD03ED6B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_3E82AD85A1D6273303C894C4CD03ED6B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_CF780182E603FD736A1394C4CD032433" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_3E82AD85A1D6273303C894C4CD03ED6B" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_QuarterlyPaymentMember" xlink:label="loc_selb_QuarterlyPaymentMember_8E151CAB174BAF7E419894C4CD03961C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_3E82AD85A1D6273303C894C4CD03ED6B" xlink:to="loc_selb_QuarterlyPaymentMember_8E151CAB174BAF7E419894C4CD03961C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_603BB3BF16F193ED8A0894C4CD030802" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_603BB3BF16F193ED8A0894C4CD030802" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_BF896166F14FCC95D6C694C4CD03F317" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_BF896166F14FCC95D6C694C4CD03F317" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_560C53E6B8D389DBDC8D94E4D351F177" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_560C53E6B8D389DBDC8D94E4D351F177" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionAmountsOfTransaction" xlink:label="loc_us-gaap_RelatedPartyTransactionAmountsOfTransaction_7C5C4D4A3E3CEC3822CF94C4CD039F8A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_RelatedPartyTransactionAmountsOfTransaction_7C5C4D4A3E3CEC3822CF94C4CD039F8A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_601CF35D74FF0C3B9EF694C4CD033539" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_601CF35D74FF0C3B9EF694C4CD033539" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_E726DCF3F12FBAF93E2194C4CD035CAD" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_E726DCF3F12FBAF93E2194C4CD035CAD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty" xlink:label="loc_us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_B8278630D285FA9729DE94C4CD03BF1E" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_638A824D5CB848F0F83994C4CD03636F" xlink:to="loc_us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_B8278630D285FA9729DE94C4CD03BF1E" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NatureOfBusinessAndBasisOfPresentation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NatureOfBusinessAndBasisOfPresentationLiquidityDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsLineItems" xlink:label="loc_us-gaap_CashAndCashEquivalentsLineItems_0D947BFF03F39E6E861497E9218ABDA6" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfCashAndCashEquivalentsTable" xlink:label="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_7843D14BFBE478D88D0B97E92188EB5B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_0D947BFF03F39E6E861497E9218ABDA6" xlink:to="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_7843D14BFBE478D88D0B97E92188EB5B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_731D05D9541BE69B022A97E9218851D9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_7843D14BFBE478D88D0B97E92188EB5B" xlink:to="loc_srt_StatementGeographicalAxis_731D05D9541BE69B022A97E9218851D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_DCCCFDDA163EDD9BFEDA97E921893FC4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_731D05D9541BE69B022A97E9218851D9" xlink:to="loc_srt_SegmentGeographicalDomain_DCCCFDDA163EDD9BFEDA97E921893FC4_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_DCCCFDDA163EDD9BFEDA97E921893FC4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_731D05D9541BE69B022A97E9218851D9" xlink:to="loc_srt_SegmentGeographicalDomain_DCCCFDDA163EDD9BFEDA97E921893FC4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_RU" xlink:label="loc_country_RU_07B2C11C0DE3254A9C1F97E92189ED2F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_DCCCFDDA163EDD9BFEDA97E921893FC4" xlink:to="loc_country_RU_07B2C11C0DE3254A9C1F97E92189ED2F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashAndCashEquivalentsAxis" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_7B0D477B7DF2334A8E8D97E921898C37" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCashAndCashEquivalentsTable_7843D14BFBE478D88D0B97E92188EB5B" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_7B0D477B7DF2334A8E8D97E921898C37" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_633F193770ADB96B140297E921897E7D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_7B0D477B7DF2334A8E8D97E921898C37" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_633F193770ADB96B140297E921897E7D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_633F193770ADB96B140297E921897E7D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_7B0D477B7DF2334A8E8D97E921898C37" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_633F193770ADB96B140297E921897E7D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_UnrestrictedCashMember" xlink:label="loc_selb_UnrestrictedCashMember_9879F7863C7D7FD549F097E921897F74" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_633F193770ADB96B140297E921897E7D" xlink:to="loc_selb_UnrestrictedCashMember_9879F7863C7D7FD549F097E921897F74" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShortTermInvestments" xlink:label="loc_us-gaap_ShortTermInvestments_E048ABD967D876085DC597E9218A83DF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_0D947BFF03F39E6E861497E9218ABDA6" xlink:to="loc_us-gaap_ShortTermInvestments_E048ABD967D876085DC597E9218A83DF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_D2BADF188F8F528C640D981500A76520" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsLineItems_0D947BFF03F39E6E861497E9218ABDA6" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_D2BADF188F8F528C640D981500A76520" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NetLossPerShare" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NetLossPerShareComputationOfBasicAndDilutedNetLossPerShareDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NetLossPerShareScheduleOfPotentialCommonSharesIssuableUponConversionOfWarrantsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_2A87F894501DC9FE0D1F69D2EDE750A9" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_587B32E1DC644E50A96D69D2EDE68DB0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_2A87F894501DC9FE0D1F69D2EDE750A9" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_587B32E1DC644E50A96D69D2EDE68DB0" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_386A0DD9D206952AC1D969D2EDE6FF59" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_587B32E1DC644E50A96D69D2EDE68DB0" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_386A0DD9D206952AC1D969D2EDE6FF59" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_386A0DD9D206952AC1D969D2EDE6FF59" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_386A0DD9D206952AC1D969D2EDE6FF59" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_A39D342503057FF3ECF069D2EDE6B288" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589" xlink:to="loc_us-gaap_EmployeeStockOptionMember_A39D342503057FF3ECF069D2EDE6B288" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_70BF1603D44DACF323AB6A1434AB5704" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_70BF1603D44DACF323AB6A1434AB5704" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_1368531725BFC811936E69D2EDE7F5DC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_B21F3DCBA4AB012602F069D2EDE6A589" xlink:to="loc_us-gaap_WarrantMember_1368531725BFC811936E69D2EDE7F5DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_DCAF1CCA88A262E1CF9369D2EDE75B5A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_2A87F894501DC9FE0D1F69D2EDE750A9" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_DCAF1CCA88A262E1CF9369D2EDE75B5A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NetLossPerShareTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/NumberOfSharesOfCommonStockPurchasedDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_9666226AC9D2DE66573297E91FB95845" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1048E9E1C45A22DCA98897E91FB61815" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_9666226AC9D2DE66573297E91FB95845" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1048E9E1C45A22DCA98897E91FB61815" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_CounterpartyNameAxis" xlink:label="loc_srt_CounterpartyNameAxis_3B95B323F237A892F74697F808FCF466" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1048E9E1C45A22DCA98897E91FB61815" xlink:to="loc_srt_CounterpartyNameAxis_3B95B323F237A892F74697F808FCF466" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_3B95B323F237A892F74697F808FCF466" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_3B95B323F237A892F74697F808FCF466" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_TimothyA.SpringerPh.D.Member" xlink:label="loc_selb_TimothyA.SpringerPh.D.Member_B719C89E4E6DA1FD9CCB97F866C6CA7E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_TimothyA.SpringerPh.D.Member_B719C89E4E6DA1FD9CCB97F866C6CA7E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EntitiesAffiliatedWithNanoDimensionMember" xlink:label="loc_selb_EntitiesAffiliatedWithNanoDimensionMember_D511396FA125BF6DC90297F947ECA3BE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_EntitiesAffiliatedWithNanoDimensionMember_D511396FA125BF6DC90297F947ECA3BE" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EntitiesAffiliatedWithOrbiMedAdvisorsMember" xlink:label="loc_selb_EntitiesAffiliatedWithOrbiMedAdvisorsMember_A8DE7171F462EE62B01C97F9482770A8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_EntitiesAffiliatedWithOrbiMedAdvisorsMember_A8DE7171F462EE62B01C97F9482770A8" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EntitiesAffiliatedWithPolarisMember" xlink:label="loc_selb_EntitiesAffiliatedWithPolarisMember_56B1D92175A855F1F1A997F94858E678" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_EntitiesAffiliatedWithPolarisMember_56B1D92175A855F1F1A997F94858E678" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SAFBNDTrustaffiliateofOmidFarokhzadM.D.Member" xlink:label="loc_selb_SAFBNDTrustaffiliateofOmidFarokhzadM.D.Member_3A1EFBA8C394A8CF730B97F948859B4B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_SAFBNDTrustaffiliateofOmidFarokhzadM.D.Member_3A1EFBA8C394A8CF730B97F948859B4B" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ChafenLuTimothySpringerswifeMember" xlink:label="loc_selb_ChafenLuTimothySpringerswifeMember_A68AE851DD1A428F6CEA97F948B657AB" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_ChafenLuTimothySpringerswifeMember_A68AE851DD1A428F6CEA97F948B657AB" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_JedSpringerTimothySpringersBrotherMember" xlink:label="loc_selb_JedSpringerTimothySpringersBrotherMember_20D4565ED9A138C8AED997F948E46F35" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_A36C761FD9A4D1B5380197F80900866C" xlink:to="loc_selb_JedSpringerTimothySpringersBrotherMember_20D4565ED9A138C8AED997F948E46F35" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_648F4FE03FE1768D617A97E91FB70BA8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_1048E9E1C45A22DCA98897E91FB61815" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_648F4FE03FE1768D617A97E91FB70BA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_DFFDD137F35C75153C6A97E91FB7950E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_648F4FE03FE1768D617A97E91FB70BA8" xlink:to="loc_us-gaap_RelatedPartyDomain_DFFDD137F35C75153C6A97E91FB7950E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_DFFDD137F35C75153C6A97E91FB7950E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_648F4FE03FE1768D617A97E91FB70BA8" xlink:to="loc_us-gaap_RelatedPartyDomain_DFFDD137F35C75153C6A97E91FB7950E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DirectorMember" xlink:label="loc_us-gaap_DirectorMember_0B541D3F8C9BD2E0B46397F79568F523" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_DFFDD137F35C75153C6A97E91FB7950E" xlink:to="loc_us-gaap_DirectorMember_0B541D3F8C9BD2E0B46397F79568F523" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_AECD8FB004862450DD9597E91FB9D3FC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_9666226AC9D2DE66573297E91FB95845" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_AECD8FB004862450DD9597E91FB9D3FC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_11A4C0A0DCCEE4626B8997E91FB9FB06" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_9666226AC9D2DE66573297E91FB95845" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_11A4C0A0DCCEE4626B8997E91FB9FB06" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3C5579289D6D2C4C9471941DDCF6160C" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_20CC205DD80B787BA2FD941DDCF57D4C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3C5579289D6D2C4C9471941DDCF6160C" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_20CC205DD80B787BA2FD941DDCF57D4C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_229A309DCCD42BE36FAD941DDCF558DE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_20CC205DD80B787BA2FD941DDCF57D4C" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_229A309DCCD42BE36FAD941DDCF558DE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_87B666ADAD5B3995D5D1941DDCF5DDAA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_229A309DCCD42BE36FAD941DDCF558DE" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_87B666ADAD5B3995D5D1941DDCF5DDAA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_87B666ADAD5B3995D5D1941DDCF5DDAA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_229A309DCCD42BE36FAD941DDCF558DE" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_87B666ADAD5B3995D5D1941DDCF5DDAA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:label="loc_us-gaap_ConstructionInProgressMember_F48C2B449D82772AE639941DDCF66E83" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_87B666ADAD5B3995D5D1941DDCF5DDAA" xlink:to="loc_us-gaap_ConstructionInProgressMember_F48C2B449D82772AE639941DDCF66E83" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ClinicalOperationsPrepaidExpenseCurrent" xlink:label="loc_selb_ClinicalOperationsPrepaidExpenseCurrent_51B829A9628AA12D738A941DDCF676DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_3C5579289D6D2C4C9471941DDCF6160C" xlink:to="loc_selb_ClinicalOperationsPrepaidExpenseCurrent_51B829A9628AA12D738A941DDCF676DC" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsNotes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PrepaidExpensesAndOtherCurrentAssetsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PropertyAndEquipment" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PropertyAndEquipmentNarrativeDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PropertyAndEquipmentScheduleOfPropertyAndEquipmentDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_505827BCFC4D55D084B1F2775402D0DC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_158E13CDB51957FDABB5C405D9ABFC5E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_505827BCFC4D55D084B1F2775402D0DC" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_158E13CDB51957FDABB5C405D9ABFC5E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83B9B3FF23D25156B7A0AA6C0D724B5B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_158E13CDB51957FDABB5C405D9ABFC5E" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83B9B3FF23D25156B7A0AA6C0D724B5B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_11A69499CAD9DEB0C85D0D296E6D2A5E_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83B9B3FF23D25156B7A0AA6C0D724B5B" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_11A69499CAD9DEB0C85D0D296E6D2A5E_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_83B9B3FF23D25156B7A0AA6C0D724B5B" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LaboratoryEquipmentMember" xlink:label="loc_selb_LaboratoryEquipmentMember_CD329F876936586DB49090446642B6E2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:to="loc_selb_LaboratoryEquipmentMember_CD329F876936586DB49090446642B6E2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="loc_us-gaap_ComputerEquipmentMember_0BA87ABAE88B5F158C069175F95809EA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:to="loc_us-gaap_ComputerEquipmentMember_0BA87ABAE88B5F158C069175F95809EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_34EB803F9CA95A52A03E7A8FA523D414" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_34EB803F9CA95A52A03E7A8FA523D414" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_A44B68F9EA565F3CA613DB81FF950794" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_A44B68F9EA565F3CA613DB81FF950794" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OfficeEquipmentMember" xlink:label="loc_us-gaap_OfficeEquipmentMember_843CEB5B0D555CAA8A0E6BE20889A68B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:to="loc_us-gaap_OfficeEquipmentMember_843CEB5B0D555CAA8A0E6BE20889A68B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:label="loc_us-gaap_ConstructionInProgressMember_B194846CE4F75039B4C2C749B3D6B6A3" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_F2F3636A3EB65D3FAE9CDB680A1C1FFE" xlink:to="loc_us-gaap_ConstructionInProgressMember_B194846CE4F75039B4C2C749B3D6B6A3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_C3699F9C90F758258D67489E3F645323" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_505827BCFC4D55D084B1F2775402D0DC" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_C3699F9C90F758258D67489E3F645323" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_EBC646C22E3859A4BE20F6023C0907D1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_505827BCFC4D55D084B1F2775402D0DC" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_EBC646C22E3859A4BE20F6023C0907D1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_DA861FDCDC4E5C98BEEB5FC54640C073" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_505827BCFC4D55D084B1F2775402D0DC" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_DA861FDCDC4E5C98BEEB5FC54640C073" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/PropertyAndEquipmentTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RelatedPartyTransactions" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RelatedPartyTransactionsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RevenueArrangements" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RevenueArrangementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_72CBB6013E710F1A62C1918E6F367346" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_srt_RangeAxis_72CBB6013E710F1A62C1918E6F367346" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_D3AB5B809DA661BA4827918E6F3685AB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_72CBB6013E710F1A62C1918E6F367346" xlink:to="loc_srt_RangeMember_D3AB5B809DA661BA4827918E6F3685AB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_D3AB5B809DA661BA4827918E6F3685AB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_72CBB6013E710F1A62C1918E6F367346" xlink:to="loc_srt_RangeMember_D3AB5B809DA661BA4827918E6F3685AB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_095AF4EB50AD1A01B667918E6F36FED3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_D3AB5B809DA661BA4827918E6F3685AB" xlink:to="loc_srt_MinimumMember_095AF4EB50AD1A01B667918E6F36FED3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_45C8E361BE299A5623A4918E6F36F932" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_D3AB5B809DA661BA4827918E6F3685AB" xlink:to="loc_srt_MaximumMember_45C8E361BE299A5623A4918E6F36F932" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RestatementAxis" xlink:label="loc_srt_RestatementAxis_3B169079B00A6D37EC04918E6F36DA8E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_srt_RestatementAxis_3B169079B00A6D37EC04918E6F36DA8E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RestatementDomain" xlink:label="loc_srt_RestatementDomain_A103CAF32057D11A233C918E6F363CC5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_3B169079B00A6D37EC04918E6F36DA8E" xlink:to="loc_srt_RestatementDomain_A103CAF32057D11A233C918E6F363CC5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RestatementDomain" xlink:label="loc_srt_RestatementDomain_A103CAF32057D11A233C918E6F363CC5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_3B169079B00A6D37EC04918E6F36DA8E" xlink:to="loc_srt_RestatementDomain_A103CAF32057D11A233C918E6F363CC5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RestatementAdjustmentMember" xlink:label="loc_srt_RestatementAdjustmentMember_AC2A2B4B0653FAF21451918E6F362914" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_A103CAF32057D11A233C918E6F363CC5" xlink:to="loc_srt_RestatementAdjustmentMember_AC2A2B4B0653FAF21451918E6F362914" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis" xlink:label="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_B62BE96177DB7DF5DAFC918E6F3606FC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_B62BE96177DB7DF5DAFC918E6F3606FC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForErrorCorrectionDomain" xlink:label="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_744CD65DBF128E69B646918E6F360D3A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_B62BE96177DB7DF5DAFC918E6F3606FC" xlink:to="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_744CD65DBF128E69B646918E6F360D3A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForErrorCorrectionDomain" xlink:label="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_744CD65DBF128E69B646918E6F360D3A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ErrorCorrectionsAndPriorPeriodAdjustmentsRestatementByRestatementPeriodAndAmountAxis_B62BE96177DB7DF5DAFC918E6F3606FC" xlink:to="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_744CD65DBF128E69B646918E6F360D3A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ErrorRecognizedUpontheInitialAdoptionofASC606Member" xlink:label="loc_selb_ErrorRecognizedUpontheInitialAdoptionofASC606Member_0FB5901D0DC46827586E918E6F3AA61E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsForErrorCorrectionDomain_744CD65DBF128E69B646918E6F360D3A" xlink:to="loc_selb_ErrorRecognizedUpontheInitialAdoptionofASC606Member_0FB5901D0DC46827586E918E6F3AA61E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_792ED76C565DE7186AD5918E6F3AE17D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_srt_ProductOrServiceAxis_792ED76C565DE7186AD5918E6F3AE17D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_297901EF32E022396732918E6F3AF476_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_792ED76C565DE7186AD5918E6F3AE17D" xlink:to="loc_srt_ProductsAndServicesDomain_297901EF32E022396732918E6F3AF476_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_297901EF32E022396732918E6F3AF476" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_792ED76C565DE7186AD5918E6F3AE17D" xlink:to="loc_srt_ProductsAndServicesDomain_297901EF32E022396732918E6F3AF476" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GrantMember" xlink:label="loc_us-gaap_GrantMember_C9B0AB91B6A50512D296918E6F3A65E6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_297901EF32E022396732918E6F3AF476" xlink:to="loc_us-gaap_GrantMember_C9B0AB91B6A50512D296918E6F3A65E6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_8D54217B4C320843FA71918E6F3AB78B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_8D54217B4C320843FA71918E6F3AB78B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_574421A3FF45861F9FDE918E6F3AD707_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8D54217B4C320843FA71918E6F3AB78B" xlink:to="loc_us-gaap_EquityComponentDomain_574421A3FF45861F9FDE918E6F3AD707_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_574421A3FF45861F9FDE918E6F3AD707" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8D54217B4C320843FA71918E6F3AB78B" xlink:to="loc_us-gaap_EquityComponentDomain_574421A3FF45861F9FDE918E6F3AD707" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_71522D3E5941FADA439B918E6F3AE425" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_574421A3FF45861F9FDE918E6F3AD707" xlink:to="loc_us-gaap_RetainedEarningsMember_71522D3E5941FADA439B918E6F3AE425" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_EF468AC80259066045CF918E6F3A94DE" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_EF468AC80259066045CF918E6F3A94DE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_DA171D39A5E3ECEA28AA918E6F3ADB06_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_EF468AC80259066045CF918E6F3A94DE" xlink:to="loc_us-gaap_TypeOfAdoptionMember_DA171D39A5E3ECEA28AA918E6F3ADB06_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_DA171D39A5E3ECEA28AA918E6F3ADB06" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_EF468AC80259066045CF918E6F3A94DE" xlink:to="loc_us-gaap_TypeOfAdoptionMember_DA171D39A5E3ECEA28AA918E6F3ADB06" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccountingStandardsUpdate201409Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201409Member_5566F26E1AE0A5181C41918E6F3EA1EC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_DA171D39A5E3ECEA28AA918E6F3ADB06" xlink:to="loc_us-gaap_AccountingStandardsUpdate201409Member_5566F26E1AE0A5181C41918E6F3EA1EC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis" xlink:label="loc_us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_617B9B3029A3F86E1C08918E6F3E7D7D" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_617B9B3029A3F86E1C08918E6F3E7D7D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProjectMember" xlink:label="loc_us-gaap_ProjectMember_9DCA41282AC69A4086BE918E6F3E057A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_617B9B3029A3F86E1C08918E6F3E7D7D" xlink:to="loc_us-gaap_ProjectMember_9DCA41282AC69A4086BE918E6F3E057A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProjectMember" xlink:label="loc_us-gaap_ProjectMember_9DCA41282AC69A4086BE918E6F3E057A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationByPropertyOrProjectAxis_617B9B3029A3F86E1C08918E6F3E7D7D" xlink:to="loc_us-gaap_ProjectMember_9DCA41282AC69A4086BE918E6F3E057A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_FirstGrantMember" xlink:label="loc_selb_FirstGrantMember_CCD52FE2ED0A1437B1AD918E6F3EA050" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProjectMember_9DCA41282AC69A4086BE918E6F3E057A" xlink:to="loc_selb_FirstGrantMember_CCD52FE2ED0A1437B1AD918E6F3EA050" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_29BCA418FA5E96719601918E6F3E9255" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_29BCA418FA5E96719601918E6F3E9255" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_29BCA418FA5E96719601918E6F3E9255" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_29BCA418FA5E96719601918E6F3E9255" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_FirstAcquisitionRightMember" xlink:label="loc_selb_FirstAcquisitionRightMember_1CD5A9A3D998FC68B500918E6F3EB7E0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:to="loc_selb_FirstAcquisitionRightMember_1CD5A9A3D998FC68B500918E6F3EB7E0" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_RussiaVasedDevelopmentFundOfNewTechnologiesDevelopmentAndCommercializationCenterGrantAgreementMember" xlink:label="loc_selb_RussiaVasedDevelopmentFundOfNewTechnologiesDevelopmentAndCommercializationCenterGrantAgreementMember_0EB9B60AB0FC789EBBB5918E6F3E4F14" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:to="loc_selb_RussiaVasedDevelopmentFundOfNewTechnologiesDevelopmentAndCommercializationCenterGrantAgreementMember_0EB9B60AB0FC789EBBB5918E6F3E4F14" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SecondAcquisitionRightMember" xlink:label="loc_selb_SecondAcquisitionRightMember_3D854C6B42B6CB8992C8918E6F3EBE3A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:to="loc_selb_SecondAcquisitionRightMember_3D854C6B42B6CB8992C8918E6F3EBE3A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LicenseAgreementTermsMember" xlink:label="loc_us-gaap_LicenseAgreementTermsMember_E0021253030B2C03248B918E6F3E6EB1" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:to="loc_us-gaap_LicenseAgreementTermsMember_E0021253030B2C03248B918E6F3E6EB1" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NationalInstitutesOfHealthGrantAgreementMember" xlink:label="loc_selb_NationalInstitutesOfHealthGrantAgreementMember_43B4ED299EE942B5BA2F918E6F3E3BF0" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:to="loc_selb_NationalInstitutesOfHealthGrantAgreementMember_43B4ED299EE942B5BA2F918E6F3E3BF0" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NationalInstituteOnDrugAbuseMember" xlink:label="loc_selb_NationalInstituteOnDrugAbuseMember_F1601AF8021F81354745918E6F42C48B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_DC364E9EC65BF62D8CEC918E6F3E130D" xlink:to="loc_selb_NationalInstituteOnDrugAbuseMember_F1601AF8021F81354745918E6F42C48B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_8A9F143755080831CBA4918E6F42BEAA" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_C08DCDAB58D379D27AE5918E6F327766" xlink:to="loc_us-gaap_TypeOfArrangementAxis_8A9F143755080831CBA4918E6F42BEAA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_8A9F143755080831CBA4918E6F42BEAA" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_8A9F143755080831CBA4918E6F42BEAA" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SparkLetterAgreementMember" xlink:label="loc_selb_SparkLetterAgreementMember_F7171DB5AE2FFEB4D93A918E6F42F4FB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_SparkLetterAgreementMember_F7171DB5AE2FFEB4D93A918E6F42F4FB" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember" xlink:label="loc_selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember_B0B643D56F7347A0D022918E6F423865" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember_B0B643D56F7347A0D022918E6F423865" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ConsiderationForPromisesMember" xlink:label="loc_selb_ConsiderationForPromisesMember_76163DEFCF0ADD3F42E5918E6F420609" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_ConsiderationForPromisesMember_76163DEFCF0ADD3F42E5918E6F420609" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DevelopmentMilestonesMember" xlink:label="loc_selb_DevelopmentMilestonesMember_9A33584C68FE998950AA918E6F420A13" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_DevelopmentMilestonesMember_9A33584C68FE998950AA918E6F420A13" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_RegulatoryMilestonesMember" xlink:label="loc_selb_RegulatoryMilestonesMember_E70E46852026FC3E39D3918E6F42911B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_RegulatoryMilestonesMember_E70E46852026FC3E39D3918E6F42911B" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_CommercialMilestonesMember" xlink:label="loc_selb_CommercialMilestonesMember_3C8167FD1498691DBE30918E6F426D3F" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_CommercialMilestonesMember_3C8167FD1498691DBE30918E6F426D3F" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndSupplyObligationMember" xlink:label="loc_selb_LicenseAndSupplyObligationMember_18F9D147FB3EFAD097EA918E6F420172" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_LicenseAndSupplyObligationMember_18F9D147FB3EFAD097EA918E6F420172" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DiscountOnOptionObligationMember" xlink:label="loc_selb_DiscountOnOptionObligationMember_62A8984AE88E0CFEA06F918E6F42A5E9" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_selb_DiscountOnOptionObligationMember_62A8984AE88E0CFEA06F918E6F42A5E9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LicenseAgreementTermsMember" xlink:label="loc_us-gaap_LicenseAgreementTermsMember_6D3656381F6BE333F16F918E6F468CEB" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_FCEA9D2999C5D4E5B2E3918E6F4257A2" xlink:to="loc_us-gaap_LicenseAgreementTermsMember_6D3656381F6BE333F16F918E6F468CEB" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementUpfrontCashPayment" xlink:label="loc_selb_LicenseAndOptionAgreementUpfrontCashPayment_41AA346B8B5B927A32C6918E6F4643AC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementUpfrontCashPayment_41AA346B8B5B927A32C6918E6F4643AC" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementAggregateAdditionalPaymentsDueWithinTwelveMonthsOfContractDate" xlink:label="loc_selb_LicenseAndOptionAgreementAggregateAdditionalPaymentsDueWithinTwelveMonthsOfContractDate_D6DD2168C9D71C0597A3918E6F467497" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementAggregateAdditionalPaymentsDueWithinTwelveMonthsOfContractDate_D6DD2168C9D71C0597A3918E6F467497" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_TermLoanNumberofPayments" xlink:label="loc_selb_TermLoanNumberofPayments_D0CD02C3A01A439DC485918E6F461133" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_TermLoanNumberofPayments_D0CD02C3A01A439DC485918E6F461133" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementTwoPaymentsDueWithinTwelveMonthsOfContractDate" xlink:label="loc_selb_LicenseAndOptionAgreementTwoPaymentsDueWithinTwelveMonthsOfContractDate_D6815A7A884C25D1026E918E6F469B4D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementTwoPaymentsDueWithinTwelveMonthsOfContractDate_D6815A7A884C25D1026E918E6F469B4D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementNumberOfAdditionalTargetsForWhichOtherPartyMayExerciseOptionsForAdditionalLicenses" xlink:label="loc_selb_LicenseAndOptionAgreementNumberOfAdditionalTargetsForWhichOtherPartyMayExerciseOptionsForAdditionalLicenses_1914BF9C636414A92D6B918E6F46978A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementNumberOfAdditionalTargetsForWhichOtherPartyMayExerciseOptionsForAdditionalLicenses_1914BF9C636414A92D6B918E6F46978A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementVariableFeeToBeReceivedForEachAdditionalTargetOptionElected" xlink:label="loc_selb_LicenseAndOptionAgreementVariableFeeToBeReceivedForEachAdditionalTargetOptionElected_921EF2D465E96F046657918E6F46582A" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementVariableFeeToBeReceivedForEachAdditionalTargetOptionElected_921EF2D465E96F046657918E6F46582A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementPeriodForOtherPartyToExerciseOptionsForAdditionalLicenses" xlink:label="loc_selb_LicenseAndOptionAgreementPeriodForOtherPartyToExerciseOptionsForAdditionalLicenses_82A79513EECBE7D9F704918E6F466508" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementPeriodForOtherPartyToExerciseOptionsForAdditionalLicenses_82A79513EECBE7D9F704918E6F466508" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementAdditionalDevelopmentAndRegulatoryMilestonePaymentsThatMayBeReceived" xlink:label="loc_selb_LicenseAndOptionAgreementAdditionalDevelopmentAndRegulatoryMilestonePaymentsThatMayBeReceived_3CF1E9B73F766F7443CC918E6F46C845" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementAdditionalDevelopmentAndRegulatoryMilestonePaymentsThatMayBeReceived_3CF1E9B73F766F7443CC918E6F46C845" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementCommercializationMilestonePaymentsThatMayBeReceived" xlink:label="loc_selb_LicenseAndOptionAgreementCommercializationMilestonePaymentsThatMayBeReceived_F284DD5F4E7DCBF920D7918E6F463689" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementCommercializationMilestonePaymentsThatMayBeReceived_F284DD5F4E7DCBF920D7918E6F463689" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementSalesMilestonePaymentsThatMayBeReceived" xlink:label="loc_selb_LicenseAndOptionAgreementSalesMilestonePaymentsThatMayBeReceived_0EFE5E2A9913920BD967918E6F46864A" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementSalesMilestonePaymentsThatMayBeReceived_0EFE5E2A9913920BD967918E6F46864A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_RevenueNumberOfPerformanceObligations" xlink:label="loc_selb_RevenueNumberOfPerformanceObligations_A0242176A3735B04B819918E6F4A2922" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_RevenueNumberOfPerformanceObligations_A0242176A3735B04B819918E6F4A2922" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EligiblePeriodToReceiveRoyalties" xlink:label="loc_selb_EligiblePeriodToReceiveRoyalties_B2E5D37D6003B36CD8B8918E6F4A5F13" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_EligiblePeriodToReceiveRoyalties_B2E5D37D6003B36CD8B8918E6F4A5F13" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PeriodForPriorWrittenNoticeToTerminateLicense" xlink:label="loc_selb_PeriodForPriorWrittenNoticeToTerminateLicense_1CA95DCA1E54AECDF41A918E6F4A9FC5" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_PeriodForPriorWrittenNoticeToTerminateLicense_1CA95DCA1E54AECDF41A918E6F4A9FC5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_1CFE6F6BFF5FBD4A1523918E6F4A4FA6" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_1CFE6F6BFF5FBD4A1523918E6F4A4FA6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_018AC16C2D0BC098F6F5918E6F4AC6AE" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_018AC16C2D0BC098F6F5918E6F4AC6AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_EF81CF719106B16170D7918E6F4A117A" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_EF81CF719106B16170D7918E6F4A117A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PremiumOfAverageDailyVolumeWeightedAveragePriceOfCommonStockPercent" xlink:label="loc_selb_PremiumOfAverageDailyVolumeWeightedAveragePriceOfCommonStockPercent_D8657C2DFCB0F444E0CA918E6F4A5837" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_PremiumOfAverageDailyVolumeWeightedAveragePriceOfCommonStockPercent_D8657C2DFCB0F444E0CA918E6F4A5837" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerLiability" xlink:label="loc_us-gaap_ContractWithCustomerLiability_1C9EA9B8392DBC9F6366918E6F4AED82" xlink:type="locator" />
    <link:definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_ContractWithCustomerLiability_1C9EA9B8392DBC9F6366918E6F4AED82" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_67256E75FAEF394BAEFD918E6F4A6677" xlink:type="locator" />
    <link:definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_67256E75FAEF394BAEFD918E6F4A6677" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_4E9888763390481E175F918E6F4A1A1D" xlink:type="locator" />
    <link:definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_4E9888763390481E175F918E6F4A1A1D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ConsecutiveCalendarDaysLeadingUpToContractDate" xlink:label="loc_selb_ConsecutiveCalendarDaysLeadingUpToContractDate_B7BD21D5CA07F3FCB4F6918E6F4A7E7F" xlink:type="locator" />
    <link:definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_ConsecutiveCalendarDaysLeadingUpToContractDate_B7BD21D5CA07F3FCB4F6918E6F4A7E7F" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_AggregatePurchasePrice" xlink:label="loc_selb_AggregatePurchasePrice_49837153C59C18BA8BEA918E6F4A4C6D" xlink:type="locator" />
    <link:definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_AggregatePurchasePrice_49837153C59C18BA8BEA918E6F4A4C6D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_StockPurchaseAgreementAuthorizedAmountOfStockToSell" xlink:label="loc_selb_StockPurchaseAgreementAuthorizedAmountOfStockToSell_B94F1D06921C2E898F6A918E6F4E2989" xlink:type="locator" />
    <link:definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_StockPurchaseAgreementAuthorizedAmountOfStockToSell_B94F1D06921C2E898F6A918E6F4E2989" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_StockPurchaseProgramNumberOfSharesAuthorizedToBeSold" xlink:label="loc_selb_StockPurchaseProgramNumberOfSharesAuthorizedToBeSold_FA09217EE1CA63F3BB76918E6F4E6B38" xlink:type="locator" />
    <link:definitionArc order="24" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_StockPurchaseProgramNumberOfSharesAuthorizedToBeSold_FA09217EE1CA63F3BB76918E6F4E6B38" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementAdditionalSubLicensePaymentReceivedLiability" xlink:label="loc_selb_LicenseAndOptionAgreementAdditionalSubLicensePaymentReceivedLiability_12111FD6E749F8DB7E46918E6F4EF918" xlink:type="locator" />
    <link:definitionArc order="25" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementAdditionalSubLicensePaymentReceivedLiability_12111FD6E749F8DB7E46918E6F4EF918" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement" xlink:label="loc_selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement_38C8CFEC85829A9A72C6918E6F4E916F" xlink:type="locator" />
    <link:definitionArc order="26" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement_38C8CFEC85829A9A72C6918E6F4E916F" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NumberOfAgreementsCombinedIntoSingleAgreementForEvaluation" xlink:label="loc_selb_NumberOfAgreementsCombinedIntoSingleAgreementForEvaluation_B574A3F010922EB52CA3918E6F4EC27F" xlink:type="locator" />
    <link:definitionArc order="27" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_NumberOfAgreementsCombinedIntoSingleAgreementForEvaluation_B574A3F010922EB52CA3918E6F4EC27F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValuePurchaseOfAssets" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValuePurchaseOfAssets_56872468CD74647D88C4918E6F4EE163" xlink:type="locator" />
    <link:definitionArc order="28" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValuePurchaseOfAssets_56872468CD74647D88C4918E6F4EE163" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_6107C00AE73AB1AC76A4918E6F4EB4C3" xlink:type="locator" />
    <link:definitionArc order="29" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_6107C00AE73AB1AC76A4918E6F4EB4C3" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_RevenueRemainingPerformanceObligationEachOption" xlink:label="loc_selb_RevenueRemainingPerformanceObligationEachOption_63BDA76A978678D3EE3B918E6F4EBA73" xlink:type="locator" />
    <link:definitionArc order="30" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_RevenueRemainingPerformanceObligationEachOption_63BDA76A978678D3EE3B918E6F4EBA73" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CapitalizedContractCostNet" xlink:label="loc_us-gaap_CapitalizedContractCostNet_C0DBF1EBD73C371E11D8918E6F4ED075" xlink:type="locator" />
    <link:definitionArc order="31" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_CapitalizedContractCostNet_C0DBF1EBD73C371E11D8918E6F4ED075" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PercentageOfSublicenseIncome" xlink:label="loc_selb_PercentageOfSublicenseIncome_B857A0535C89BF4B8C9B918E6F4EC97A" xlink:type="locator" />
    <link:definitionArc order="32" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_PercentageOfSublicenseIncome_B857A0535C89BF4B8C9B918E6F4EC97A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaap_AssetsCurrent_634D868897806E8428D3918E6F4EC96A" xlink:type="locator" />
    <link:definitionArc order="33" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_AssetsCurrent_634D868897806E8428D3918E6F4EC96A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AssetsNoncurrent" xlink:label="loc_us-gaap_AssetsNoncurrent_D6C87B3BDBAF2E0FC3F9918E6F4E347D" xlink:type="locator" />
    <link:definitionArc order="34" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_AssetsNoncurrent_D6C87B3BDBAF2E0FC3F9918E6F4E347D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_EB563E8C4FDE59769F49918E6F4EEDA3" xlink:type="locator" />
    <link:definitionArc order="35" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_EB563E8C4FDE59769F49918E6F4EEDA3" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ContractwithCustomerLiabilityDeferredRevenueRevenueRecognized" xlink:label="loc_selb_ContractwithCustomerLiabilityDeferredRevenueRevenueRecognized_65AD0DF093040755088E918E6F4E2BE4" xlink:type="locator" />
    <link:definitionArc order="36" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_ContractwithCustomerLiabilityDeferredRevenueRevenueRecognized_65AD0DF093040755088E918E6F4E2BE4" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_StockPurchaseProgramAggregatePurchasePricePerShare" xlink:label="loc_selb_StockPurchaseProgramAggregatePurchasePricePerShare_D46EB9F8B5FDE35757D4918E6F52A01B" xlink:type="locator" />
    <link:definitionArc order="37" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_StockPurchaseProgramAggregatePurchasePricePerShare_D46EB9F8B5FDE35757D4918E6F52A01B" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_GrantsFromEntitiesAmountOfGrantAward" xlink:label="loc_selb_GrantsFromEntitiesAmountOfGrantAward_ABB3D88E3F27415705CD918E6F52B50C" xlink:type="locator" />
    <link:definitionArc order="38" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_GrantsFromEntitiesAmountOfGrantAward_ABB3D88E3F27415705CD918E6F52B50C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PercentageOfEstimatedCostOfResearchPlan" xlink:label="loc_selb_PercentageOfEstimatedCostOfResearchPlan_CC5A393236AD8DEED9D6918E6F5270A5" xlink:type="locator" />
    <link:definitionArc order="39" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_PercentageOfEstimatedCostOfResearchPlan_CC5A393236AD8DEED9D6918E6F5270A5" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_RemainingPercentageContributedByEntity" xlink:label="loc_selb_RemainingPercentageContributedByEntity_F3AAD0674355E42FDC42918E6F522096" xlink:type="locator" />
    <link:definitionArc order="40" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_RemainingPercentageContributedByEntity_F3AAD0674355E42FDC42918E6F522096" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_IncreaseDecreaseInContingentlyRepayableGrantFunding" xlink:label="loc_selb_IncreaseDecreaseInContingentlyRepayableGrantFunding_A04F1A63D8F3415400F0918E6F522FBD" xlink:type="locator" />
    <link:definitionArc order="41" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_IncreaseDecreaseInContingentlyRepayableGrantFunding_A04F1A63D8F3415400F0918E6F522FBD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_8C824E89A8C1BC00C302918E6F526A4A" xlink:type="locator" />
    <link:definitionArc order="42" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_8C824E89A8C1BC00C302918E6F526A4A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_GrantsFromEntitiesUnusedGrantFunds" xlink:label="loc_selb_GrantsFromEntitiesUnusedGrantFunds_699303F30D051D640B99918E6F520437" xlink:type="locator" />
    <link:definitionArc order="43" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_GrantsFromEntitiesUnusedGrantFunds_699303F30D051D640B99918E6F520437" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_OverspentCosts" xlink:label="loc_selb_OverspentCosts_BD883EF9AB958EF05915918E6F52A2B7" xlink:type="locator" />
    <link:definitionArc order="44" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_OverspentCosts_BD883EF9AB958EF05915918E6F52A2B7" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PeriodToAuditExpenditureIncurred" xlink:label="loc_selb_PeriodToAuditExpenditureIncurred_00BC4B5857C7C9C3496D918E6F52EA99" xlink:type="locator" />
    <link:definitionArc order="45" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_PeriodToAuditExpenditureIncurred_00BC4B5857C7C9C3496D918E6F52EA99" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_3941E559937893C2DAB9918E6F52339E" xlink:type="locator" />
    <link:definitionArc order="46" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_3941E559937893C2DAB9918E6F52339E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ReimbursementInvoice" xlink:label="loc_selb_ReimbursementInvoice_043D8B4B3B6794864B2A918E6F52A931" xlink:type="locator" />
    <link:definitionArc order="47" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_selb_ReimbursementInvoice_043D8B4B3B6794864B2A918E6F52A931" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherLiabilities" xlink:label="loc_us-gaap_OtherLiabilities_9192970B4A1DA0FBBB82918E6F566F16" xlink:type="locator" />
    <link:definitionArc order="48" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_681F142B0567B2F29BCC918E6F46EC07" xlink:to="loc_us-gaap_OtherLiabilities_9192970B4A1DA0FBBB82918E6F566F16" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RevenueArrangementsScheduleOfChangesInContractLiabilitiesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RevenueArrangementsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/RevenueArrangementsTransactionPriceAllocatedToFuturePerformanceObligationDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_41990D0842745FFC8C6B6D42C09A0CC2" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_0AF0CB3A41735F288A903ED53D0E1AEE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_41990D0842745FFC8C6B6D42C09A0CC2" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_0AF0CB3A41735F288A903ED53D0E1AEE" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_C2F8FB71B0B85D09BCD83C660C41E34C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_0AF0CB3A41735F288A903ED53D0E1AEE" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_C2F8FB71B0B85D09BCD83C660C41E34C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_5FE296D7558C0A1C4817DF6C58F574AD_920EE169FFE95E169740A8F1C48B2013" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_C2F8FB71B0B85D09BCD83C660C41E34C" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_5FE296D7558C0A1C4817DF6C58F574AD_920EE169FFE95E169740A8F1C48B2013" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_C7213FFFE5CD5847AAFD1BB50C9B79A3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_C2F8FB71B0B85D09BCD83C660C41E34C" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_C7213FFFE5CD5847AAFD1BB50C9B79A3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LicenseAgreementTermsMember" xlink:label="loc_us-gaap_LicenseAgreementTermsMember_BDB14041DE3F5F41B238225440AF8AE5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_C7213FFFE5CD5847AAFD1BB50C9B79A3" xlink:to="loc_us-gaap_LicenseAgreementTermsMember_BDB14041DE3F5F41B238225440AF8AE5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_5AFD4FEC0CE95DF5BE7962A8EC0962AD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_0AF0CB3A41735F288A903ED53D0E1AEE" xlink:to="loc_us-gaap_TypeOfArrangementAxis_5AFD4FEC0CE95DF5BE7962A8EC0962AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_938273D52D9F2C572CFDDF6C58F652CE_920EE169FFE95E169740A8F1C48B2013" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_5AFD4FEC0CE95DF5BE7962A8EC0962AD" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_938273D52D9F2C572CFDDF6C58F652CE_920EE169FFE95E169740A8F1C48B2013" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_ADA0F9474BED5A689015C11C9F5A970E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_5AFD4FEC0CE95DF5BE7962A8EC0962AD" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_ADA0F9474BED5A689015C11C9F5A970E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndSupplyObligationMember" xlink:label="loc_selb_LicenseAndSupplyObligationMember_746921F171D153468FB00544A80D2EFB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_ADA0F9474BED5A689015C11C9F5A970E" xlink:to="loc_selb_LicenseAndSupplyObligationMember_746921F171D153468FB00544A80D2EFB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_AC2D9C2040235996938038A42A84AE16" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_0AF0CB3A41735F288A903ED53D0E1AEE" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_AC2D9C2040235996938038A42A84AE16" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_C0B0D37CDB8D52D489BFFAA94AD8CB8D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_41990D0842745FFC8C6B6D42C09A0CC2" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_C0B0D37CDB8D52D489BFFAA94AD8CB8D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlans" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansAdditionalInformationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="loc_us-gaap_TitleOfIndividualAxis_D128FE018D274E306E35990F0561FFF0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:to="loc_us-gaap_TitleOfIndividualAxis_D128FE018D274E306E35990F0561FFF0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1509B196FED9E1C8F44F990F056204E4_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TitleOfIndividualAxis_D128FE018D274E306E35990F0561FFF0" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1509B196FED9E1C8F44F990F056204E4_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1509B196FED9E1C8F44F990F056204E4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TitleOfIndividualAxis_D128FE018D274E306E35990F0561FFF0" xlink:to="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1509B196FED9E1C8F44F990F056204E4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ExecutiveOfficerMember" xlink:label="loc_us-gaap_ExecutiveOfficerMember_6024B69BD10A2EEA6C1B990F0562B4DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TitleOfIndividualWithRelationshipToEntityDomain_1509B196FED9E1C8F44F990F056204E4" xlink:to="loc_us-gaap_ExecutiveOfficerMember_6024B69BD10A2EEA6C1B990F0562B4DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VestingAxis" xlink:label="loc_us-gaap_VestingAxis_8244FE33092F4389EE2A990F0562E0A4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:to="loc_us-gaap_VestingAxis_8244FE33092F4389EE2A990F0562E0A4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VestingDomain" xlink:label="loc_us-gaap_VestingDomain_E53205792DB03D0F089C990F05628ACA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_8244FE33092F4389EE2A990F0562E0A4" xlink:to="loc_us-gaap_VestingDomain_E53205792DB03D0F089C990F05628ACA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_VestingDomain" xlink:label="loc_us-gaap_VestingDomain_E53205792DB03D0F089C990F05628ACA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_8244FE33092F4389EE2A990F0562E0A4" xlink:to="loc_us-gaap_VestingDomain_E53205792DB03D0F089C990F05628ACA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_39A1E024B8D562955006990F0562F51F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_E53205792DB03D0F089C990F05628ACA" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_39A1E024B8D562955006990F0562F51F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_1F93E113BD5B784DEF67990F0563B5C3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_E53205792DB03D0F089C990F05628ACA" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_1F93E113BD5B784DEF67990F0563B5C3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_0C250303D7CF8E41195D990F05630E3B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:to="loc_us-gaap_PlanNameAxis_0C250303D7CF8E41195D990F05630E3B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_0C250303D7CF8E41195D990F05630E3B" xlink:to="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_0C250303D7CF8E41195D990F05630E3B" xlink:to="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_The2008PlanAndThe2016PlanMember" xlink:label="loc_selb_The2008PlanAndThe2016PlanMember_431A26C3CE941CAB252C990F0563CE71" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205" xlink:to="loc_selb_The2008PlanAndThe2016PlanMember_431A26C3CE941CAB252C990F0563CE71" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_StockIncentivePlan2008Member" xlink:label="loc_selb_StockIncentivePlan2008Member_BDB6B02DD33315B4F53D990F056432EA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_The2008PlanAndThe2016PlanMember_431A26C3CE941CAB252C990F0563CE71" xlink:to="loc_selb_StockIncentivePlan2008Member_BDB6B02DD33315B4F53D990F056432EA" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_StockIncentivePlan2016Member" xlink:label="loc_selb_StockIncentivePlan2016Member_6CDA2ABA6E38525E0D51990F0564E7FC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_The2008PlanAndThe2016PlanMember_431A26C3CE941CAB252C990F0563CE71" xlink:to="loc_selb_StockIncentivePlan2016Member_6CDA2ABA6E38525E0D51990F0564E7FC" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EmploymentInducementIncentiveAwardPlanMember" xlink:label="loc_selb_EmploymentInducementIncentiveAwardPlanMember_26EA2449839FC126B691990F0564CD8F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205" xlink:to="loc_selb_EmploymentInducementIncentiveAwardPlanMember_26EA2449839FC126B691990F0564CD8F" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EmployeeStockPurchasePlan2016Member" xlink:label="loc_selb_EmployeeStockPurchasePlan2016Member_D4C371DF15CC6B655E03990F0564BC2B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_E01C4C9FAB6BDFD7F286990F0563F205" xlink:to="loc_selb_EmployeeStockPurchasePlan2016Member_D4C371DF15CC6B655E03990F0564BC2B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_5294CD2BE46BDFC82BAD990F056491B2" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:to="loc_us-gaap_AwardTypeAxis_5294CD2BE46BDFC82BAD990F056491B2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5294CD2BE46BDFC82BAD990F056491B2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5294CD2BE46BDFC82BAD990F056491B2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_D0CB79C20DF81D4F13AD990F0565A806" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200" xlink:to="loc_us-gaap_StockCompensationPlanMember_D0CB79C20DF81D4F13AD990F0565A806" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NonEmployeeStockOptionMember" xlink:label="loc_selb_NonEmployeeStockOptionMember_239C9D26010A5A68A793990F0565F95E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200" xlink:to="loc_selb_NonEmployeeStockOptionMember_239C9D26010A5A68A793990F0565F95E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_F7BADBBD3DAAC0130760990F056543FD" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_D2C89DE2482656C1EE94990F05655200" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_F7BADBBD3DAAC0130760990F056543FD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_04CE2A25D12231AEE142990F056601E5" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7D98C0BB2C7DF3E0C1F7990F05613251" xlink:to="loc_srt_RangeAxis_04CE2A25D12231AEE142990F056601E5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_977A47F89708D798C013990F05663588_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_04CE2A25D12231AEE142990F056601E5" xlink:to="loc_srt_RangeMember_977A47F89708D798C013990F05663588_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_977A47F89708D798C013990F05663588" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_04CE2A25D12231AEE142990F056601E5" xlink:to="loc_srt_RangeMember_977A47F89708D798C013990F05663588" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_682EE01E7137A943B030990F0566C908" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_977A47F89708D798C013990F05663588" xlink:to="loc_srt_MinimumMember_682EE01E7137A943B030990F0566C908" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_8D6F51C4CBF7C350DF86990F056C1C9C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_977A47F89708D798C013990F05663588" xlink:to="loc_srt_MaximumMember_8D6F51C4CBF7C350DF86990F056C1C9C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_BEA383EF3E53F00DF289990F056C22E4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_BEA383EF3E53F00DF289990F056C22E4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_C954EECC85DB634D64B3990F056CB5C2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_C954EECC85DB634D64B3990F056CB5C2" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardVotingRightsOfCommonStockHoldingPercentage" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardVotingRightsOfCommonStockHoldingPercentage_9824690B7CEEE5E38D9E990F056C72CA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardVotingRightsOfCommonStockHoldingPercentage_9824690B7CEEE5E38D9E990F056C72CA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_E55A58FB56C9316727EE990F056D17F4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_E55A58FB56C9316727EE990F056D17F4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_96BBD96591592B1B9FA7990F056D5514" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_96BBD96591592B1B9FA7990F056D5514" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EmployeeStockPurchasePlanMaximumPercentageOfSharesReservedBasedOnSharesOutstanding" xlink:label="loc_selb_EmployeeStockPurchasePlanMaximumPercentageOfSharesReservedBasedOnSharesOutstanding_369721FC026898610A4E990F056D6BA2" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_selb_EmployeeStockPurchasePlanMaximumPercentageOfSharesReservedBasedOnSharesOutstanding_369721FC026898610A4E990F056D6BA2" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease_22D168E9606BA22CA05B990F056D31D9" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease_22D168E9606BA22CA05B990F056D31D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_986E89AA3157CCFB61D7990F056D84DE" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_986E89AA3157CCFB61D7990F056D84DE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_A18686852168033E0D3C990F056EB624" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_A18686852168033E0D3C990F056EB624" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease_F87D708AC8D59C368689990F8BC61356" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease_F87D708AC8D59C368689990F8BC61356" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_835406E0F140A92C9277990F056E5A0B" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_835406E0F140A92C9277990F056E5A0B" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate_7772ED6FB7ECEA772AFC990F056EADB5" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardForfeituresRate_7772ED6FB7ECEA772AFC990F056EADB5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_DD1D01612D15BA93F67B990F056E89BA" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_DD1D01612D15BA93F67B990F056E89BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1CBEEB4540405CA7FED0990F056EF305" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_1CBEEB4540405CA7FED0990F056EF305" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_2056966E2B417C71BCA3990F056E90FD" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_2056966E2B417C71BCA3990F056E90FD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_710FB8E0215ED61957E4990F056E1BA8" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_710FB8E0215ED61957E4990F056E1BA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_B6A910E27485A5B5F2BA990F056F0761" xlink:type="locator" />
    <link:definitionArc order="17" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_B6A910E27485A5B5F2BA990F056F0761" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_9FB408082F779836A177990F056FF5CF" xlink:type="locator" />
    <link:definitionArc order="18" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_9FB408082F779836A177990F056FF5CF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_ACBE3E339D7F5BBA82CC990F056F6E2B" xlink:type="locator" />
    <link:definitionArc order="19" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_ACBE3E339D7F5BBA82CC990F056F6E2B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber_FCF6F6351CBF853048CD990F056FC4B2" xlink:type="locator" />
    <link:definitionArc order="20" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber_FCF6F6351CBF853048CD990F056FC4B2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_4D4673899A33D04356C4990F056F5CA7" xlink:type="locator" />
    <link:definitionArc order="21" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_4D4673899A33D04356C4990F056F5CA7" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedGoodsAndNonEmployeeServicesTransactionCompensationNotYetRecognizedStockOptions" xlink:label="loc_selb_ShareBasedGoodsAndNonEmployeeServicesTransactionCompensationNotYetRecognizedStockOptions_333DDD0B39CC8C1C612C990F056F8E3E" xlink:type="locator" />
    <link:definitionArc order="22" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_selb_ShareBasedGoodsAndNonEmployeeServicesTransactionCompensationNotYetRecognizedStockOptions_333DDD0B39CC8C1C612C990F056F8E3E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedGoodsAndNonEmployeeServicesTransactionCompensationNotYetRecognizedStockOptionsPeriod" xlink:label="loc_selb_ShareBasedGoodsAndNonEmployeeServicesTransactionCompensationNotYetRecognizedStockOptionsPeriod_B378C43C63EA67E1960C990F057004BC" xlink:type="locator" />
    <link:definitionArc order="23" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_selb_ShareBasedGoodsAndNonEmployeeServicesTransactionCompensationNotYetRecognizedStockOptionsPeriod_B378C43C63EA67E1960C990F057004BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_212BBD1FF6287BCBA654990F057078D3" xlink:type="locator" />
    <link:definitionArc order="24" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_212BBD1FF6287BCBA654990F057078D3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_1B9A700C4F68841CE7F0990F057024B5" xlink:type="locator" />
    <link:definitionArc order="25" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_1B9A700C4F68841CE7F0990F057024B5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_7CC9AA87539392B52D6C990F05708E10" xlink:type="locator" />
    <link:definitionArc order="26" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_7CC9AA87539392B52D6C990F05708E10" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_C41BEDC44AC883B259BA990F05703A02" xlink:type="locator" />
    <link:definitionArc order="27" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_C41BEDC44AC883B259BA990F05703A02" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_6CD3BF41B00B2E19E217990F0571F397" xlink:type="locator" />
    <link:definitionArc order="28" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_6CD3BF41B00B2E19E217990F0571F397" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_26A2D16B4232429F2F36990F05719262" xlink:type="locator" />
    <link:definitionArc order="29" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_33328F46EE003A41B9AD990F056C225A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_26A2D16B4232429F2F36990F05719262" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansEmployeeStockPurchasePlanNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_22A423EF9D8ECA3774B76AD35706631A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_22A423EF9D8ECA3774B76AD35706631A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_BB1317AC61825266B1016AD35706DC59" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_22A423EF9D8ECA3774B76AD35706631A" xlink:to="loc_us-gaap_PlanNameAxis_BB1317AC61825266B1016AD35706DC59" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_1CA9E1F096C0D96ED7E26AD357069A1B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_BB1317AC61825266B1016AD35706DC59" xlink:to="loc_us-gaap_PlanNameDomain_1CA9E1F096C0D96ED7E26AD357069A1B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_1CA9E1F096C0D96ED7E26AD357069A1B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_BB1317AC61825266B1016AD35706DC59" xlink:to="loc_us-gaap_PlanNameDomain_1CA9E1F096C0D96ED7E26AD357069A1B" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EmployeeStockPurchasePlan2016Member" xlink:label="loc_selb_EmployeeStockPurchasePlan2016Member_1BBCBBCAF4CCA1B1748B6AD3570730A0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_1CA9E1F096C0D96ED7E26AD357069A1B" xlink:to="loc_selb_EmployeeStockPurchasePlan2016Member_1BBCBBCAF4CCA1B1748B6AD3570730A0" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NumberOfOfferingPeriods" xlink:label="loc_selb_NumberOfOfferingPeriods_364312328CAAECA249A96AD3570879CF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_selb_NumberOfOfferingPeriods_364312328CAAECA249A96AD3570879CF" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_OfferingPeriod" xlink:label="loc_selb_OfferingPeriod_C36D662ECCF0EE10835F6AD35708E494" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_selb_OfferingPeriod_C36D662ECCF0EE10835F6AD35708E494" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_9D8EC7EBE434B972F1026AD357080F27" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_9D8EC7EBE434B972F1026AD357080F27" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_A05A63FF1B834C90C9D16AD35708E5E4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_A05A63FF1B834C90C9D16AD35708E5E4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_108CD7D353A8BFCE57666AD357086B4A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_108CD7D353A8BFCE57666AD357086B4A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_EmployeeStockPurchasePlanMaximumPercentageOfSharesReservedBasedOnSharesOutstanding" xlink:label="loc_selb_EmployeeStockPurchasePlanMaximumPercentageOfSharesReservedBasedOnSharesOutstanding_07EF359DA41FB6D1395C6AD35708F124" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_selb_EmployeeStockPurchasePlanMaximumPercentageOfSharesReservedBasedOnSharesOutstanding_07EF359DA41FB6D1395C6AD35708F124" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease_1725441F550816E000E76AD357085D1A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantIncreaseDecrease_1725441F550816E000E76AD357085D1A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_331E1AEBB9BE468901326AD35708C1BC" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod_331E1AEBB9BE468901326AD35708C1BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_15BF0F6EC8388A109E166AD35708C0A2" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6630A5B151C0E8C9C7B56AD35708F737" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_15BF0F6EC8388A109E166AD35708C0A2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansRestrictedStockUnitsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_26AA3D9BD066FA3F8E9D94C4CE8861F0" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_05B120891342C076C97D94C4CE84A539" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_26AA3D9BD066FA3F8E9D94C4CE8861F0" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_05B120891342C076C97D94C4CE84A539" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_2F0EAD20A189477A3F3D94C4CE841593" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_05B120891342C076C97D94C4CE84A539" xlink:to="loc_us-gaap_AwardTypeAxis_2F0EAD20A189477A3F3D94C4CE841593" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BCA5076319011C34BD994C4CE84438C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_2F0EAD20A189477A3F3D94C4CE841593" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BCA5076319011C34BD994C4CE84438C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BCA5076319011C34BD994C4CE84438C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_2F0EAD20A189477A3F3D94C4CE841593" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BCA5076319011C34BD994C4CE84438C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_E2AF0FCC6BD8FE06491B94C4CE84303F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1BCA5076319011C34BD994C4CE84438C" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_E2AF0FCC6BD8FE06491B94C4CE84303F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_26AA3D9BD066FA3F8E9D94C4CE8861F0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_A2DBDBE511F46131A8F894C4CE886646" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_A2DBDBE511F46131A8F894C4CE886646" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_4962D9A9DE1D65ED571194C4CE88FC3C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_4962D9A9DE1D65ED571194C4CE88FC3C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_750FC0C1015B6D03FECF94C4CE8879E8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_750FC0C1015B6D03FECF94C4CE8879E8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease_9AD3FB4692F1605817F494C4CE88BED1" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecrease_9AD3FB4692F1605817F494C4CE88BED1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_59EA4DC10E285B7964E594C4CE880A3A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_285048B66978FC87FD7794C4CE88A8B3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_59EA4DC10E285B7964E594C4CE880A3A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_26AA3D9BD066FA3F8E9D94C4CE8861F0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_F96E793AA1964EE16ECF94C4CE88BC6A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_F96E793AA1964EE16ECF94C4CE88BC6A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_E8EE9D925775CAB8971694C4CE882D09" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_E8EE9D925775CAB8971694C4CE882D09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_0A6D40A528EB26A1FA3F94C4CE886F09" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_0A6D40A528EB26A1FA3F94C4CE886F09" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsPeriodIncreaseDecreaseWeightedAverageGrantDateFairValue" xlink:label="loc_selb_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsPeriodIncreaseDecreaseWeightedAverageGrantDateFairValue_5BC35551282106E36F6C94C4CE88DBF5" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:to="loc_selb_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsPeriodIncreaseDecreaseWeightedAverageGrantDateFairValue_5BC35551282106E36F6C94C4CE88DBF5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_0EE6DAC02E4AD68A630F94C4CE883C0F" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_12DDB5C6C39EA775F8F394C4CE889103" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_0EE6DAC02E4AD68A630F94C4CE883C0F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_7C50706B66EE888382A894C4CE889D0B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_26AA3D9BD066FA3F8E9D94C4CE8861F0" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_7C50706B66EE888382A894C4CE889D0B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_1FBA77EDD6098D013EB394C4CE8880CA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_26AA3D9BD066FA3F8E9D94C4CE8861F0" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_1FBA77EDD6098D013EB394C4CE8880CA" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansScheduleOfAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3AEEEC8818C9AA9BE9D594C4CE9DFD0D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3AEEEC8818C9AA9BE9D594C4CE9DFD0D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_2DF097877C1A5430DA6094C4CE9E5496" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_3AEEEC8818C9AA9BE9D594C4CE9DFD0D" xlink:to="loc_us-gaap_AwardTypeAxis_2DF097877C1A5430DA6094C4CE9E5496" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F93EAA3FF298162D8B6794C4CE9E76BF_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_2DF097877C1A5430DA6094C4CE9E5496" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F93EAA3FF298162D8B6794C4CE9E76BF_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F93EAA3FF298162D8B6794C4CE9E76BF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_2DF097877C1A5430DA6094C4CE9E5496" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F93EAA3FF298162D8B6794C4CE9E76BF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_F4CF56306AD687AE9B6A94C4CE9E8FAE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F93EAA3FF298162D8B6794C4CE9E76BF" xlink:to="loc_us-gaap_StockCompensationPlanMember_F4CF56306AD687AE9B6A94C4CE9E8FAE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_2120993AB533BA94048494C4CE9E5B29" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_2120993AB533BA94048494C4CE9E5B29" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_08F3A4E46EAABF664AD394C4CE9EAE08" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_08F3A4E46EAABF664AD394C4CE9EAE08" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_82857EB67D339D19D17E94C4CE9E2E28" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_82857EB67D339D19D17E94C4CE9E2E28" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_380E344C5357A016458994C4CE9E8F3D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_380E344C5357A016458994C4CE9E8F3D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue_942833622601F0B616B494C4CE9E3491" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_4072C47EEBCD0B26989B94C4CE9E7861" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageFairValue_942833622601F0B616B494C4CE9E3491" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansScheduleOfStockBasedCompensationExpenseRelatedToStockOptionsAndRestrictedCommonStockDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_6445BEB8FDD01990CD896AD35243725D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9DD65A718358A4D09E896AD352382565" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_6445BEB8FDD01990CD896AD35243725D" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9DD65A718358A4D09E896AD352382565" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_DAF8121EAD98A8125B166AD3523A4618" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9DD65A718358A4D09E896AD352382565" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_DAF8121EAD98A8125B166AD3523A4618" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_21EE1BE421738733265C6AD3523A972D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_DAF8121EAD98A8125B166AD3523A4618" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_21EE1BE421738733265C6AD3523A972D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_21EE1BE421738733265C6AD3523A972D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_DAF8121EAD98A8125B166AD3523A4618" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_21EE1BE421738733265C6AD3523A972D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ResearchAndDevelopmentMember" xlink:label="loc_selb_ResearchAndDevelopmentMember_014FC51F0CD34D8F22396AD3523AF658" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_21EE1BE421738733265C6AD3523A972D" xlink:to="loc_selb_ResearchAndDevelopmentMember_014FC51F0CD34D8F22396AD3523AF658" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_262082482E46B7A3112E6AD35243A967" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_21EE1BE421738733265C6AD3523A972D" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_262082482E46B7A3112E6AD35243A967" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_B0EEA4454BD1E7ABB1E56AD35243800D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_6445BEB8FDD01990CD896AD35243725D" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_B0EEA4454BD1E7ABB1E56AD35243800D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansSummaryOfActivityUnder2008PlanAnd2016PlanDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6991CEB259AC4736319E940FD70B5A8E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6991CEB259AC4736319E940FD70B5A8E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_2CFD486B15256F0DF436940FD70BCF68" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6991CEB259AC4736319E940FD70B5A8E" xlink:to="loc_us-gaap_PlanNameAxis_2CFD486B15256F0DF436940FD70BCF68" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_AB08D8377C6F6911BE0D940FD70B83E7_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_2CFD486B15256F0DF436940FD70BCF68" xlink:to="loc_us-gaap_PlanNameDomain_AB08D8377C6F6911BE0D940FD70B83E7_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_AB08D8377C6F6911BE0D940FD70B83E7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_2CFD486B15256F0DF436940FD70BCF68" xlink:to="loc_us-gaap_PlanNameDomain_AB08D8377C6F6911BE0D940FD70B83E7" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_The2008PlanAndThe2016PlanMember" xlink:label="loc_selb_The2008PlanAndThe2016PlanMember_49D0C5B2A344712CFAA5940FD70C1B7C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_AB08D8377C6F6911BE0D940FD70B83E7" xlink:to="loc_selb_The2008PlanAndThe2016PlanMember_49D0C5B2A344712CFAA5940FD70C1B7C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_53F481A8FA72239077A4940FD70CF475" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6991CEB259AC4736319E940FD70B5A8E" xlink:to="loc_us-gaap_AwardTypeAxis_53F481A8FA72239077A4940FD70CF475" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_FA50F7733155D14BF5D1940FD70C9B81_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_53F481A8FA72239077A4940FD70CF475" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_FA50F7733155D14BF5D1940FD70C9B81_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_FA50F7733155D14BF5D1940FD70C9B81" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_53F481A8FA72239077A4940FD70CF475" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_FA50F7733155D14BF5D1940FD70C9B81" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_791C91A2E41B192E9075940FD70C6CC8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_FA50F7733155D14BF5D1940FD70C9B81" xlink:to="loc_us-gaap_StockCompensationPlanMember_791C91A2E41B192E9075940FD70C6CC8" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NonEmployeeStockOptionMember" xlink:label="loc_selb_NonEmployeeStockOptionMember_DE7C29845E6992797E1A940FD70D4524" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_FA50F7733155D14BF5D1940FD70C9B81" xlink:to="loc_selb_NonEmployeeStockOptionMember_DE7C29845E6992797E1A940FD70D4524" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_30737F09C1973727FAD2940FD70DF496" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_30737F09C1973727FAD2940FD70DF496" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_33AF845AB638AB832886940FD70DA82D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_30737F09C1973727FAD2940FD70DF496" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_33AF845AB638AB832886940FD70DA82D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_6F2016345897F66F5633940FD70EA72C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_30737F09C1973727FAD2940FD70DF496" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_6F2016345897F66F5633940FD70EA72C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_A362797FA139ABFCC6EE940FD70EB3DB" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_30737F09C1973727FAD2940FD70DF496" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_A362797FA139ABFCC6EE940FD70EB3DB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_26C838ED1BCD0C90C5CF940FD70F13CB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_30737F09C1973727FAD2940FD70DF496" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_26C838ED1BCD0C90C5CF940FD70F13CB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_30332BEFB0FCDE3933DC940FD70FD430" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_30332BEFB0FCDE3933DC940FD70FD430" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_013317A77F509EFCCE31940FD70F993B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_013317A77F509EFCCE31940FD70F993B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_D42F5A6698C733557E48940FD70F2708" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_D42F5A6698C733557E48940FD70F2708" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_EA945E75122A68BCC0E5940FD70F23AD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_D42F5A6698C733557E48940FD70F2708" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_EA945E75122A68BCC0E5940FD70F23AD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_01FEC8D0E1BE3F13AFCF940FD70F3C88" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_D42F5A6698C733557E48940FD70F2708" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_01FEC8D0E1BE3F13AFCF940FD70F3C88" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_D93A2DD2B48BF2EB2816940FD710D1C1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_D42F5A6698C733557E48940FD70F2708" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_D93A2DD2B48BF2EB2816940FD710D1C1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_B136CD0371E2EB6C6E4F940FD7106FF7" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_D42F5A6698C733557E48940FD70F2708" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_B136CD0371E2EB6C6E4F940FD7106FF7" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice" xlink:label="loc_selb_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice_28E3072C3739CAC7AA21940FD7108063" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_selb_SharebasedCompensationArrangementbySharebasedPaymentAwardOptionsVestedWeightedAverageExercisePrice_28E3072C3739CAC7AA21940FD7108063" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_DFF1B72C1F8799E50825940FD7102CF5" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_DFF1B72C1F8799E50825940FD7102CF5" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_D6F4938EA1D54714ED78940FD710E30F" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_D6F4938EA1D54714ED78940FD710E30F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_A1C66E4FE680AF7CD0E2940FD710AE1E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_D6F4938EA1D54714ED78940FD710E30F" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_A1C66E4FE680AF7CD0E2940FD710AE1E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm" xlink:label="loc_selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm_1BC20C49CA911975F2FC940FD712C10B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_D6F4938EA1D54714ED78940FD710E30F" xlink:to="loc_selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm_1BC20C49CA911975F2FC940FD712C10B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_E9E208C94FACA5684CD5940FD712D61A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_D6F4938EA1D54714ED78940FD710E30F" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_E9E208C94FACA5684CD5940FD712D61A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract" xlink:label="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_D2B15CE8179AEC505C26940FD71279A8" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0BA03352461E37AB0125940FD70DD30B" xlink:to="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_D2B15CE8179AEC505C26940FD71279A8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_A26EAE7B55BF136D357B940FD7120437" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_D2B15CE8179AEC505C26940FD71279A8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_A26EAE7B55BF136D357B940FD7120437" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue" xlink:label="loc_selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue_77B6A53D2CC30AA8FF00940FD713C9A4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_D2B15CE8179AEC505C26940FD71279A8" xlink:to="loc_selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingIntrinsicValue_77B6A53D2CC30AA8FF00940FD713C9A4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_6C9BBC92024A4BE8ABBB940FD713CDE8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValueAbstract_D2B15CE8179AEC505C26940FD71279A8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_6C9BBC92024A4BE8ABBB940FD713CDE8" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockIncentivePlansTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockholdersEquity" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsidiarySaleOfStockLineItems" xlink:label="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable" xlink:label="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_DEBF4441A4AC8DF5C31D94C4CEFD46F8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_DEBF4441A4AC8DF5C31D94C4CEFD46F8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_D959C1CBC34816273CCD94C4CEFD4F74" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_DEBF4441A4AC8DF5C31D94C4CEFD46F8" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_D959C1CBC34816273CCD94C4CEFD4F74" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_D959C1CBC34816273CCD94C4CEFD4F74" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_D959C1CBC34816273CCD94C4CEFD4F74" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PublicStockOfferingMember" xlink:label="loc_selb_PublicStockOfferingMember_A2D5D0D57A53191E724194C4CEFD49BC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_selb_PublicStockOfferingMember_A2D5D0D57A53191E724194C4CEFD49BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="loc_us-gaap_OverAllotmentOptionMember_844E5E7E52890A77553494C4CEFD3897" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_us-gaap_OverAllotmentOptionMember_844E5E7E52890A77553494C4CEFD3897" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_AtTheMarketOfferingMember" xlink:label="loc_selb_AtTheMarketOfferingMember_85684E684024A38FFE5B94C4CEFD3EF4" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_selb_AtTheMarketOfferingMember_85684E684024A38FFE5B94C4CEFD3EF4" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_A2017PIPEMember" xlink:label="loc_selb_A2017PIPEMember_15D57F71A2B1086AF1D094C4CEFD22F6" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_selb_A2017PIPEMember_15D57F71A2B1086AF1D094C4CEFD22F6" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SpringerPurchaseAgreementMember" xlink:label="loc_selb_SpringerPurchaseAgreementMember_7A72D11B47085E4B828E94C4CEFD3827" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_selb_SpringerPurchaseAgreementMember_7A72D11B47085E4B828E94C4CEFD3827" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_A8170E5562F6ED4C3C6E94C4CEFD7F09" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_us-gaap_WarrantMember_A8170E5562F6ED4C3C6E94C4CEFD7F09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_IPOMember" xlink:label="loc_us-gaap_IPOMember_2B6D397DD75EEA3E9BD294C4CF01615D" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_F84F795F4CD829BA804894C4CEFD573F" xlink:to="loc_us-gaap_IPOMember_2B6D397DD75EEA3E9BD294C4CF01615D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaap_StatementClassOfStockAxis_7C56CE2B75D4D749FD8B94C4CF016E9A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsidiaryOrEquityMethodInvesteeSaleOfStockBySubsidiaryOrEquityInvesteeTable_DEBF4441A4AC8DF5C31D94C4CEFD46F8" xlink:to="loc_us-gaap_StatementClassOfStockAxis_7C56CE2B75D4D749FD8B94C4CF016E9A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_779CA877908EEBCE16A894C4CF01DE67_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_7C56CE2B75D4D749FD8B94C4CF016E9A" xlink:to="loc_us-gaap_ClassOfStockDomain_779CA877908EEBCE16A894C4CF01DE67_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaap_ClassOfStockDomain_779CA877908EEBCE16A894C4CF01DE67" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_7C56CE2B75D4D749FD8B94C4CF016E9A" xlink:to="loc_us-gaap_ClassOfStockDomain_779CA877908EEBCE16A894C4CF01DE67" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_D3339EE6B4AA6A69276394C4CF01ACE4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_779CA877908EEBCE16A894C4CF01DE67" xlink:to="loc_us-gaap_CommonStockMember_D3339EE6B4AA6A69276394C4CF01ACE4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_EBFDCFDFEEC95ECE786394C4CF03690B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_EBFDCFDFEEC95ECE786394C4CF03690B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_1461280E2A8228C23DD794C4CF0388B3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_1461280E2A8228C23DD794C4CF0388B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_8A368736D9AA07D33BB394C4CF03E72C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_8A368736D9AA07D33BB394C4CF03E72C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PublicSecuritiesOfferingAmountAuthorized" xlink:label="loc_selb_PublicSecuritiesOfferingAmountAuthorized_5CD931C0CA9388C13CBB94C4CF0306A2" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_selb_PublicSecuritiesOfferingAmountAuthorized_5CD931C0CA9388C13CBB94C4CF0306A2" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PublicStockOfferingAmountAuthorized" xlink:label="loc_selb_PublicStockOfferingAmountAuthorized_BA07034E1D19508EE86694C4CF03BE10" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_selb_PublicStockOfferingAmountAuthorized_BA07034E1D19508EE86694C4CF03BE10" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_1656A93DD1A87B3F78E494C4CF03708C" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_1656A93DD1A87B3F78E494C4CF03708C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_A919489B798C4AE37A3B94C4CF03DC62" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_A919489B798C4AE37A3B94C4CF03DC62" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_C167CC8BCBFE10349D6F94C4CF03E087" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_C167CC8BCBFE10349D6F94C4CF03E087" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ClassOfWarrantsOrRightsTerm" xlink:label="loc_selb_ClassOfWarrantsOrRightsTerm_A366C06FED3AB4A36B0A94C4CF03695E" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_selb_ClassOfWarrantsOrRightsTerm_A366C06FED3AB4A36B0A94C4CF03695E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ClassofWarrantorRightIssuedtoPurchaseCommonStock" xlink:label="loc_selb_ClassofWarrantorRightIssuedtoPurchaseCommonStock_FC000DCA7991EA1D6B2E94C4CF03C4FE" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_selb_ClassofWarrantorRightIssuedtoPurchaseCommonStock_FC000DCA7991EA1D6B2E94C4CF03C4FE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_79985740B6A4704FEE7194C4CF04C9F8" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_79985740B6A4704FEE7194C4CF04C9F8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:label="loc_us-gaap_ProceedsFromIssuanceInitialPublicOffering_BD904C5338282B835FB094C4CF0417D5" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_ProceedsFromIssuanceInitialPublicOffering_BD904C5338282B835FB094C4CF0417D5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:label="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_0111A700C22111D97AD794C4CF04D4A5" xlink:type="locator" />
    <link:definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_0111A700C22111D97AD794C4CF04D4A5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_60FCE5F81E91452AC0D194C4CF044720" xlink:type="locator" />
    <link:definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_60FCE5F81E91452AC0D194C4CF044720" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_VotesPerShareofCommonStock" xlink:label="loc_selb_VotesPerShareofCommonStock_CB304D8DD267D5358EBC94C4CF04E70C" xlink:type="locator" />
    <link:definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_selb_VotesPerShareofCommonStock_CB304D8DD267D5358EBC94C4CF04E70C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_DividendsCommonStock" xlink:label="loc_us-gaap_DividendsCommonStock_1A50988B704D42274DA594C4CF05A22F" xlink:type="locator" />
    <link:definitionArc order="16" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsidiarySaleOfStockLineItems_4FF697CCAD441118E75E94C4CF010EED" xlink:to="loc_us-gaap_DividendsCommonStock_1A50988B704D42274DA594C4CF05A22F" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockholdersEquityScheduleOfAuthorizedSharesOfCommonStockForFutureIssuanceDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaap_ClassOfStockLineItems_5CD26BE4142026093626918E703B51C7" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_EBC9416FD8AE3A12B2EA918E70375107" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_5CD26BE4142026093626918E703B51C7" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_EBC9416FD8AE3A12B2EA918E70375107" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_0B0A97EED72CEBF01F40918E7037052E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_EBC9416FD8AE3A12B2EA918E70375107" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_0B0A97EED72CEBF01F40918E7037052E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_977F212C77398A835251918E703BDB20_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_0B0A97EED72CEBF01F40918E7037052E" xlink:to="loc_us-gaap_EquityComponentDomain_977F212C77398A835251918E703BDB20_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_977F212C77398A835251918E703BDB20" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_0B0A97EED72CEBF01F40918E7037052E" xlink:to="loc_us-gaap_EquityComponentDomain_977F212C77398A835251918E703BDB20" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_WarrantMember" xlink:label="loc_us-gaap_WarrantMember_87B6AB247548C7B74156918E703B471B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_977F212C77398A835251918E703BDB20" xlink:to="loc_us-gaap_WarrantMember_87B6AB247548C7B74156918E703B471B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_2B8C0555D3B36A5D8158918E703BD2E8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_EBC9416FD8AE3A12B2EA918E70375107" xlink:to="loc_us-gaap_AwardTypeAxis_2B8C0555D3B36A5D8158918E703BD2E8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_2B8C0555D3B36A5D8158918E703BD2E8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_2B8C0555D3B36A5D8158918E703BD2E8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_31164A18E65ABD345638918E703BD9F7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48" xlink:to="loc_us-gaap_StockCompensationPlanMember_31164A18E65ABD345638918E703BD9F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_C01CC254CC260519930591E012166BB7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48" xlink:to="loc_us-gaap_RestrictedStockMember_C01CC254CC260519930591E012166BB7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_9D9FA0981F8ECCD14D07918E703B0420" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_9D9FA0981F8ECCD14D07918E703B0420" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4982AAB5D0B24D8CAB95918E703BBAEB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_C82D8E0D54BFA4B5DDD3918E703B6B48" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4982AAB5D0B24D8CAB95918E703BBAEB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_9B42E83F7C7ECC02DE99918E703B2036" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_5CD26BE4142026093626918E703B51C7" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_9B42E83F7C7ECC02DE99918E703B2036" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/StockholdersEquityTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesCashEquivalentsShortTermInvestmentsAndRestrictedCashDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesComponentsOfAccumulatedOtherComprehensiveIncomeLossNetOfTaxDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E7AA2BFCBA566730FE9593F560A25A1E" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_5B912D16322178EB657893F5609B94C8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E7AA2BFCBA566730FE9593F560A25A1E" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_5B912D16322178EB657893F5609B94C8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_8391771D87C5D5B573FE93F5609B678E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_5B912D16322178EB657893F5609B94C8" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_8391771D87C5D5B573FE93F5609B678E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8391771D87C5D5B573FE93F5609B678E" xlink:to="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8391771D87C5D5B573FE93F5609B678E" xlink:to="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedTranslationAdjustmentMember" xlink:label="loc_us-gaap_AccumulatedTranslationAdjustmentMember_B34F676DD67BB8CC521693F560A1DC83" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D" xlink:to="loc_us-gaap_AccumulatedTranslationAdjustmentMember_B34F676DD67BB8CC521693F560A1DC83" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_C636A28F1735833272C893F560A191DA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_C636A28F1735833272C893F560A191DA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_3EB210EFCF2E9B6B84B593F560A10725" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_7F28ECF24DBEF7D6370D93F5609B586D" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_3EB210EFCF2E9B6B84B593F560A10725" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_AOCIAttributableToParentNetOfTaxRollForward" xlink:label="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_BE48E44ED6EF1A031CFC93F560A210FB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_E7AA2BFCBA566730FE9593F560A25A1E" xlink:to="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_BE48E44ED6EF1A031CFC93F560A210FB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_BCA3BA44BE804901945B93F560A2CF7F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_BE48E44ED6EF1A031CFC93F560A210FB" xlink:to="loc_us-gaap_StockholdersEquity_BCA3BA44BE804901945B93F560A2CF7F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_5DE3F96841C1C75DE9B393FCF45477EB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_BE48E44ED6EF1A031CFC93F560A210FB" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_5DE3F96841C1C75DE9B393FCF45477EB" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesConcentrationsOfCreditRiskAndOffBalanceSheetRiskDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConcentrationRiskLineItems" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_D2BFEC62F36A5C0A88C553EF84B9F384" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ConcentrationRiskTable" xlink:label="loc_us-gaap_ConcentrationRiskTable_002EAF2E72C45D87B8E2E639EACF8371" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_D2BFEC62F36A5C0A88C553EF84B9F384" xlink:to="loc_us-gaap_ConcentrationRiskTable_002EAF2E72C45D87B8E2E639EACF8371" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_2CDE2FDC03255F9DA81E8B21B1F10509" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_002EAF2E72C45D87B8E2E639EACF8371" xlink:to="loc_srt_StatementGeographicalAxis_2CDE2FDC03255F9DA81E8B21B1F10509" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_343BCFD16DC85CE4D5D70D2972461782_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_2CDE2FDC03255F9DA81E8B21B1F10509" xlink:to="loc_srt_SegmentGeographicalDomain_343BCFD16DC85CE4D5D70D2972461782_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_9FB1F88A889259429BFE4BA1C8D13ACB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_2CDE2FDC03255F9DA81E8B21B1F10509" xlink:to="loc_srt_SegmentGeographicalDomain_9FB1F88A889259429BFE4BA1C8D13ACB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_RU" xlink:label="loc_country_RU_E0FF6E280AD15C84BC259F23CA079F19" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_9FB1F88A889259429BFE4BA1C8D13ACB" xlink:to="loc_country_RU_E0FF6E280AD15C84BC259F23CA079F19" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_CurrencyAxis" xlink:label="loc_srt_CurrencyAxis_885AFE0D26D55BBEAFF23B1B7404D84A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_002EAF2E72C45D87B8E2E639EACF8371" xlink:to="loc_srt_CurrencyAxis_885AFE0D26D55BBEAFF23B1B7404D84A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_27BE33C8BD53A44CA42B0D297246F466_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CurrencyAxis_885AFE0D26D55BBEAFF23B1B7404D84A" xlink:to="loc_currency_AllCurrenciesDomain_27BE33C8BD53A44CA42B0D297246F466_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_B576B2B6749D56B9B37C07ED4536357B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CurrencyAxis_885AFE0D26D55BBEAFF23B1B7404D84A" xlink:to="loc_currency_AllCurrenciesDomain_B576B2B6749D56B9B37C07ED4536357B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd#currency_USD" xlink:label="loc_currency_USD_A574B869A29C5225B9F6204E4F5B8971" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_B576B2B6749D56B9B37C07ED4536357B" xlink:to="loc_currency_USD_A574B869A29C5225B9F6204E4F5B8971" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_Cash" xlink:label="loc_us-gaap_Cash_748A87025E085148B72821360EBF080D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_D2BFEC62F36A5C0A88C553EF84B9F384" xlink:to="loc_us-gaap_Cash_748A87025E085148B72821360EBF080D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesImpairmentOfLonglivedAssetsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesPropertyAndEquipmentDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_EC62171CFDC68899761D69D2EE148D2F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_8FD56B191D6849BA29E969D2EE128B13" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_EC62171CFDC68899761D69D2EE148D2F" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_8FD56B191D6849BA29E969D2EE128B13" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_623F92404E6C33D5A8C569D2EE128B68" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_8FD56B191D6849BA29E969D2EE128B13" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_623F92404E6C33D5A8C569D2EE128B68" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_623F92404E6C33D5A8C569D2EE128B68" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_623F92404E6C33D5A8C569D2EE128B68" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_9AF06FF05EF4989244B169D2EE13DA1F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_9AF06FF05EF4989244B169D2EE13DA1F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_EquipmentMember" xlink:label="loc_us-gaap_EquipmentMember_60B197A1A391A506701569D2EE13478B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834" xlink:to="loc_us-gaap_EquipmentMember_60B197A1A391A506701569D2EE13478B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="loc_us-gaap_ComputerEquipmentMember_9BBC0F0D3254D58DC5C569D2EE1480B9" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_04784A54D33578ED282A69D2EE13F834" xlink:to="loc_us-gaap_ComputerEquipmentMember_9BBC0F0D3254D58DC5C569D2EE1480B9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_D9BED39A623525255B3769D2EE143F30" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_EC62171CFDC68899761D69D2EE148D2F" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_D9BED39A623525255B3769D2EE143F30" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesScheduleOfReconciliationOfCashCashEquivalentsAndRestrictedCashDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesSegmentInformationDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/TechnologyLicenseAgreements" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.selectabio.com/role/TechnologyLicenseAgreementsMassachusettsEyeAndEarInfirmaryAndSchepensEyeResearchInstituteIncNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_40678BE048EF519CA8EBD392BAE892BB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_40678BE048EF519CA8EBD392BAE892BB" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_EF28F7E24DB25BD98BF5DE2472F41A2C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_40678BE048EF519CA8EBD392BAE892BB" xlink:to="loc_us-gaap_TypeOfArrangementAxis_EF28F7E24DB25BD98BF5DE2472F41A2C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_216ABAEAFD5C44BB464BE681F622970B_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_EF28F7E24DB25BD98BF5DE2472F41A2C" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_216ABAEAFD5C44BB464BE681F622970B_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E9AC4FC784FB5146B736F21207A5233A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_EF28F7E24DB25BD98BF5DE2472F41A2C" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E9AC4FC784FB5146B736F21207A5233A" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAgreementWithMassachusettsEyeAndEarInfirmaryAndSchepensEyeResearchInstituteInc.Member" xlink:label="loc_selb_LicenseAgreementWithMassachusettsEyeAndEarInfirmaryAndSchepensEyeResearchInstituteInc.Member_59FB9EB1662E5CB19D2CCB3FC3CC3D46" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_E9AC4FC784FB5146B736F21207A5233A" xlink:to="loc_selb_LicenseAgreementWithMassachusettsEyeAndEarInfirmaryAndSchepensEyeResearchInstituteInc.Member_59FB9EB1662E5CB19D2CCB3FC3CC3D46" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_B5122FE6EA1756549712BC538977C4D1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_40678BE048EF519CA8EBD392BAE892BB" xlink:to="loc_srt_RangeAxis_B5122FE6EA1756549712BC538977C4D1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_2F6BA20C876E333FC75DE681F62623A6_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_B5122FE6EA1756549712BC538977C4D1" xlink:to="loc_srt_RangeMember_2F6BA20C876E333FC75DE681F62623A6_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_55AC70610CDA5B59894DDF0C3FCB3FE9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_B5122FE6EA1756549712BC538977C4D1" xlink:to="loc_srt_RangeMember_55AC70610CDA5B59894DDF0C3FCB3FE9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_7423FF92AD1D54988FF409BF0655AC49" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_55AC70610CDA5B59894DDF0C3FCB3FE9" xlink:to="loc_srt_MinimumMember_7423FF92AD1D54988FF409BF0655AC49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_00FE7AE4C0735C6AAF5D533CCACBFA8D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_55AC70610CDA5B59894DDF0C3FCB3FE9" xlink:to="loc_srt_MaximumMember_00FE7AE4C0735C6AAF5D533CCACBFA8D" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_NumberOfProductsMarketedAndSold" xlink:label="loc_selb_NumberOfProductsMarketedAndSold_73AE8C7345395CD0A0C6F5C20282DB3C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:to="loc_selb_NumberOfProductsMarketedAndSold_73AE8C7345395CD0A0C6F5C20282DB3C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_DevelopmentMilestoneMethodAmountPaid" xlink:label="loc_selb_DevelopmentMilestoneMethodAmountPaid_A376ED075A3858C38C07E0879F6BA795" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:to="loc_selb_DevelopmentMilestoneMethodAmountPaid_A376ED075A3858C38C07E0879F6BA795" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_SalesMilestoneMethodAmountPaid" xlink:label="loc_selb_SalesMilestoneMethodAmountPaid_692C4E83669F573A9B5BAAE0049273FF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:to="loc_selb_SalesMilestoneMethodAmountPaid_692C4E83669F573A9B5BAAE0049273FF" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_InitialLicenseFeePaid" xlink:label="loc_selb_InitialLicenseFeePaid_667F1A844D885BEE8B51C4CB78A85551" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:to="loc_selb_InitialLicenseFeePaid_667F1A844D885BEE8B51C4CB78A85551" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_AccruedLicenseFees" xlink:label="loc_selb_AccruedLicenseFees_9A864F2D8F9F5DD0B97A5FA727ED5887" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_48846D4F0B1E5F6790E46231CCC15692" xlink:to="loc_selb_AccruedLicenseFees_9A864F2D8F9F5DD0B97A5FA727ED5887" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/TechnologyLicenseAgreementsMitNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_7A654BD259A05FA3965D60F30883F589" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_4350D6815C1C5517A8DA4F5CA1861EBD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_7A654BD259A05FA3965D60F30883F589" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_4350D6815C1C5517A8DA4F5CA1861EBD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_DC24697BB9CD56D89981E74B367C7B69" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_4350D6815C1C5517A8DA4F5CA1861EBD" xlink:to="loc_us-gaap_TypeOfArrangementAxis_DC24697BB9CD56D89981E74B367C7B69" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_EBB0C9385F360592547B0D29746A4265_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_DC24697BB9CD56D89981E74B367C7B69" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_EBB0C9385F360592547B0D29746A4265_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_DCBCB1FF94C85F31A5EC1D4DA7AC083C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_DC24697BB9CD56D89981E74B367C7B69" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_DCBCB1FF94C85F31A5EC1D4DA7AC083C" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember" xlink:label="loc_selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember_62BF655A224254E9B5927A7A31D56376" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_DCBCB1FF94C85F31A5EC1D4DA7AC083C" xlink:to="loc_selb_ExclusivePatentLicenseAgreementWithMassachusettsInstituteOfTechnologyMember_62BF655A224254E9B5927A7A31D56376" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_3237AE3C269E5D2D92131D143E6C8C9D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_4350D6815C1C5517A8DA4F5CA1861EBD" xlink:to="loc_srt_RangeAxis_3237AE3C269E5D2D92131D143E6C8C9D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_1A8B8839AEAFDE9FF8150D29746BA588_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_3237AE3C269E5D2D92131D143E6C8C9D" xlink:to="loc_srt_RangeMember_1A8B8839AEAFDE9FF8150D29746BA588_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_89DDCDA1AEF35D35ABC33152ECBDD336" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_3237AE3C269E5D2D92131D143E6C8C9D" xlink:to="loc_srt_RangeMember_89DDCDA1AEF35D35ABC33152ECBDD336" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_B56AEF512F755929B4A16EE16D2FAACF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_89DDCDA1AEF35D35ABC33152ECBDD336" xlink:to="loc_srt_MinimumMember_B56AEF512F755929B4A16EE16D2FAACF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2018/elts/srt-2018-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_8C08FFD7E3FB55ACA6D3359F50A8BFCA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_89DDCDA1AEF35D35ABC33152ECBDD336" xlink:to="loc_srt_MaximumMember_8C08FFD7E3FB55ACA6D3359F50A8BFCA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_349080B4F43350AB96B512AE0EB239CC" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_4350D6815C1C5517A8DA4F5CA1861EBD" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_349080B4F43350AB96B512AE0EB239CC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_D9C1CED01E44189DF6410D29746CBB0F_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_349080B4F43350AB96B512AE0EB239CC" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_D9C1CED01E44189DF6410D29746CBB0F_7840FEC2B80E5514B6B28FB61A3023AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_01DDF3B7CF165427AEC3ED1D53E02780" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis_349080B4F43350AB96B512AE0EB239CC" xlink:to="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_01DDF3B7CF165427AEC3ED1D53E02780" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_LicenseAgreementTermsMember" xlink:label="loc_us-gaap_LicenseAgreementTermsMember_325ED4A4B14B591794F5C0BD65A91B31" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_01DDF3B7CF165427AEC3ED1D53E02780" xlink:to="loc_us-gaap_LicenseAgreementTermsMember_325ED4A4B14B591794F5C0BD65A91B31" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PercentageOfSublicenseIncome" xlink:label="loc_selb_PercentageOfSublicenseIncome_205018FFE0A25318A933DB48BD571B86" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_7A654BD259A05FA3965D60F30883F589" xlink:to="loc_selb_PercentageOfSublicenseIncome_205018FFE0A25318A933DB48BD571B86" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PaymentAmountUponAchievementOfDefinedClinicalMilestones" xlink:label="loc_selb_PaymentAmountUponAchievementOfDefinedClinicalMilestones_526551D6CA3B5D5C8C9D424CEF0FB503" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_7A654BD259A05FA3965D60F30883F589" xlink:to="loc_selb_PaymentAmountUponAchievementOfDefinedClinicalMilestones_526551D6CA3B5D5C8C9D424CEF0FB503" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementContractualPaymentsMade" xlink:label="loc_selb_LicenseAndOptionAgreementContractualPaymentsMade_54F771F42A05545C81BC16E7F091B30E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_7A654BD259A05FA3965D60F30883F589" xlink:to="loc_selb_LicenseAndOptionAgreementContractualPaymentsMade_54F771F42A05545C81BC16E7F091B30E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement" xlink:label="loc_selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement_E9A9C12F60F35F5EB0B64E00263C77E2" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_7A654BD259A05FA3965D60F30883F589" xlink:to="loc_selb_LicenseAndOptionAgreementCalculatedPremiumPaidForEquityInvestmentUnderPurchaseAgreement_E9A9C12F60F35F5EB0B64E00263C77E2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.selectabio.com/role/TechnologyLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_3E9321611398E4B8BF4E6AD818CCA96D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_3E9321611398E4B8BF4E6AD818CCA96D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="loc_us-gaap_TypeOfArrangementAxis_74B96A3073240A9D8E3F6AD818D06200" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_3E9321611398E4B8BF4E6AD818CCA96D" xlink:to="loc_us-gaap_TypeOfArrangementAxis_74B96A3073240A9D8E3F6AD818D06200" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_7648177BBBB8E7E844B66AD818D02DB3_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_74B96A3073240A9D8E3F6AD818D06200" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_7648177BBBB8E7E844B66AD818D02DB3_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2018/elts/us-gaap-2018-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_7648177BBBB8E7E844B66AD818D02DB3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_74B96A3073240A9D8E3F6AD818D06200" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_7648177BBBB8E7E844B66AD818D02DB3" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember" xlink:label="loc_selb_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember_81DC510DB0AF0C98C0A36AD818D4617E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_7648177BBBB8E7E844B66AD818D02DB3" xlink:to="loc_selb_LicenseAgreementWithShenyangSunshinePharmaceuticalCompanyLimitedMember_81DC510DB0AF0C98C0A36AD818D4617E" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PeriodForCommercializationOfProduct" xlink:label="loc_selb_PeriodForCommercializationOfProduct_493DFA2F5B7A0E3A2AEF6AD818D565A9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_selb_PeriodForCommercializationOfProduct_493DFA2F5B7A0E3A2AEF6AD818D565A9" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid" xlink:label="loc_selb_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid_87BD828BC7F334F8D4BE6AD818D53ADD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_selb_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid_87BD828BC7F334F8D4BE6AD818D53ADD" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingSvpTechnology" xlink:label="loc_selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingSvpTechnology_5E161B85CBC5373D28016AD818D69E76" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsContainingSvpTechnology_5E161B85CBC5373D28016AD818D69E76" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsWithoutSvpTechnology" xlink:label="loc_selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsWithoutSvpTechnology_47FE3EC8AAFEFEA47B6D6AD818D608B1" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_selb_AggregateAmountForFuturePaymentsUponAchievementOfClinicalAndRegulatoryApprovalMilestonesForProductsWithoutSvpTechnology_47FE3EC8AAFEFEA47B6D6AD818D608B1" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_MaximumPercentagePaidOnTieredRoyalties" xlink:label="loc_selb_MaximumPercentagePaidOnTieredRoyalties_7EF841D4CB01A4D0DC656AD818D69F55" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_selb_MaximumPercentagePaidOnTieredRoyalties_7EF841D4CB01A4D0DC656AD818D69F55" xlink:type="arc" />
    <link:loc xlink:href="selb-20190331.xsd#selb_PeriodForPriorWrittenNoticeToTerminateLicense" xlink:label="loc_selb_PeriodForPriorWrittenNoticeToTerminateLicense_BDCC014520D00A6CA6436AD818D8CA73" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_A8A591378AF988F7D5AA6AD818D53ADC" xlink:to="loc_selb_PeriodForPriorWrittenNoticeToTerminateLicense_BDCC014520D00A6CA6436AD818D8CA73" xlink:type="arc" />
  </link:definitionLink>
</link:linkbase>
