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  <xs:element id="selb_AstellasGeneTherapiesMember" abstract="true" name="AstellasGeneTherapiesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm" abstract="false" name="SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedOutstandingWeightedAverageRemainingContractualTerm" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="selb_AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid" abstract="false" name="AggregateAmountOfUpfrontAndMilestoneBasedPaymentsPaid" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_EmployeeStockPurchasePlan2016Member" abstract="false" name="EmployeeStockPurchasePlan2016Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementDevelopmentCostReimbursements" abstract="false" name="LicenseAndOptionAgreementDevelopmentCostReimbursements" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementFutureAdditionalPaymentsExpected" abstract="false" name="LicenseAndOptionAgreementFutureAdditionalPaymentsExpected" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" abstract="true" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="selb_ContractwithCustomerLiabilityDeferredRevenueIncreaseFromCashReceipts" abstract="false" name="ContractwithCustomerLiabilityDeferredRevenueIncreaseFromCashReceipts" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_EmploymentInducementIncentiveAwardPlanMember" abstract="true" name="EmploymentInducementIncentiveAwardPlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_A2020TermLoansMember" abstract="true" name="A2020TermLoansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_AskBioLicenseMember" abstract="true" name="AskBioLicenseMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="selb_LicenseAndOptionAgreementContractualPaymentsMade" abstract="false" name="LicenseAndOptionAgreementContractualPaymentsMade" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale" abstract="false" name="CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsAndDebtSecuritiesAvailableForSale" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="selb_LoanAndSecurityAgreementPercentageOfConsolidatedCash" abstract="false" name="LoanAndSecurityAgreementPercentageOfConsolidatedCash" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
</xs:schema>
