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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-03-11T16:55:43.5224542+00:00 -->
<!-- Version            : 5.0.1.728 -->
<!-- Package ID         : 6c4410054de34ed1893de871626bc506 -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="asmb_CashEquivalentsAtCarryingValueAndAvailableForSaleDebtSecuritiesGrossUnrealizedGain" name="CashEquivalentsAtCarryingValueAndAvailableForSaleDebtSecuritiesGrossUnrealizedGain" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_CashEquivalentsAtCarryingValueAndAvailableForSaleDebtSecuritiesGrossUnrealizedLoss" name="CashEquivalentsAtCarryingValueAndAvailableForSaleDebtSecuritiesGrossUnrealizedLoss" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_CashEquivalentsAtCarryingValueAndAvailableForSaleDebtSecuritiesEstimatedFairValue" name="CashEquivalentsAtCarryingValueAndAvailableForSaleDebtSecuritiesEstimatedFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_LongTermMarketableSecuritiesMember" name="LongTermMarketableSecuritiesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_ShortTermMarketableSecuritiesMember" name="ShortTermMarketableSecuritiesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_AccruedRestructuringChargesCurrent" name="AccruedRestructuringChargesCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_RestructuringAndRelatedCostIncurredCostBenefit" name="RestructuringAndRelatedCostIncurredCostBenefit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_EmployeeSeveranceAndRelatedBenefitsMember" name="EmployeeSeveranceAndRelatedBenefitsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_AssetImpairmentAndOtherCostsMember" name="AssetImpairmentAndOtherCostsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_RestructuringChargesForRemainingPaymentObligationsOfLeasedEquipment" name="RestructuringChargesForRemainingPaymentObligationsOfLeasedEquipment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_MicrobiomePurchaseAgreementMember" name="MicrobiomePurchaseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_RegulatoryMember" name="RegulatoryMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_PotentialMilestonePayments" name="PotentialMilestonePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_ConsiderationOfAssetsSaleAgreementToBeReceived" name="ConsiderationOfAssetsSaleAgreementToBeReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ReductionInResearchAndDevelopmentExpense" name="ReductionInResearchAndDevelopmentExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_SaleConsiderationReceivable" name="SaleConsiderationReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_TwoThousandAndTwentyAtTheMarketOfferingsMember" name="TwoThousandAndTwentyAtTheMarketOfferingsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_InitialPublicOfferingValueOfSharesAuthorized" name="InitialPublicOfferingValueOfSharesAuthorized" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_SaleOfStockPricePerWarrant" name="SaleOfStockPricePerWarrant" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_ThresholdOwnershipPercentageToExerciseWarrant" name="ThresholdOwnershipPercentageToExerciseWarrant" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_WarrantsExercisableDescription" name="WarrantsExercisableDescription" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_ProceedsFromSaleOfPreFundedWarrants" name="ProceedsFromSaleOfPreFundedWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ClassOfWarrantOrRightIssuanceDates" name="ClassOfWarrantOrRightIssuanceDates" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_ClassOfWarrantOrRightExpirationDates" name="ClassOfWarrantOrRightExpirationDates" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_WarrantsIssuedOneMember" name="WarrantsIssuedOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_NumberOfWarrantsExercised" name="NumberOfWarrantsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_EmployeesIncludingExecutiveOfficersMember" name="EmployeesIncludingExecutiveOfficersMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_TwoThousandEighteenEmployeeStockPurchasePlanMember" name="TwoThousandEighteenEmployeeStockPurchasePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedInPeriodTotalIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantedInPeriodTotalIntrinsicValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitedInPeriodTotalIntrinsicValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeitedInPeriodTotalIntrinsicValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairAmount" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_CumulativeCatchUpAdjustmentOfStockBasedCompensationExpense" name="CumulativeCatchUpAdjustmentOfStockBasedCompensationExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesPerEmployee" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesPerEmployee" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_ShareBasedCompensationArrangementFairValueAssumptionsExercisePrice" name="ShareBasedCompensationArrangementFairValueAssumptionsExercisePrice" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_CostOfReimbursableExpenses" name="CostOfReimbursableExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_BeiGeneAgreementMember" name="BeiGeneAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_DevelopmentAndRegulatoryMember" name="DevelopmentAndRegulatoryMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_NetSalesMilestonePaymentsMember" name="NetSalesMilestonePaymentsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_BeiGeneLimitedMember" name="BeiGeneLimitedMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_ArbutusAgreementMember" name="ArbutusAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_AntiosAgreementMember" name="AntiosAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_AgreedToPayMaximumAmountDevelopmentAndRegulatoryCosts" name="AgreedToPayMaximumAmountDevelopmentAndRegulatoryCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_UpfrontCashPaymentReceived" name="UpfrontCashPaymentReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_IncrementalCosts" name="IncrementalCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ClinicalTrialCollaborationCostsIncurred" name="ClinicalTrialCollaborationCostsIncurred" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_SublicensingFee" name="SublicensingFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_PerformanceMilestonePayment" name="PerformanceMilestonePayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_PaymentOfMilestone" name="PaymentOfMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_DoorsAgreementMember" name="DoorsAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_LicensesAgreementTerm" name="LicensesAgreementTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_UpFrontPayments" name="UpFrontPayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_SuccessBasedMilestonesPayment" name="SuccessBasedMilestonesPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_ResearchAndDevelopmentFundingCharges" name="ResearchAndDevelopmentFundingCharges" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_FederalCurrentAndDeferredIncomeTaxExpenseBenefit" name="FederalCurrentAndDeferredIncomeTaxExpenseBenefit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_StateCurrentAndDeferredIncomeTaxExpenseBenefit" name="StateCurrentAndDeferredIncomeTaxExpenseBenefit" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_CurrentAndDeferredFederalLocalTaxExpenseBenefitAbstract" name="CurrentAndDeferredFederalLocalTaxExpenseBenefitAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_CurrentAndDeferredStateLocalTaxExpenseBenefitAbstract" name="CurrentAndDeferredStateLocalTaxExpenseBenefitAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_EffectiveIncomeTaxRateReconciliationReturnToProvisionAdjustments" name="EffectiveIncomeTaxRateReconciliationReturnToProvisionAdjustments" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_EffectiveIncomeTaxRateReconciliationUncertainTaxPositionsRate" name="EffectiveIncomeTaxRateReconciliationUncertainTaxPositionsRate" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_EffectiveIncomeTaxRateReconciliationImpairmentOfGoodwill" name="EffectiveIncomeTaxRateReconciliationImpairmentOfGoodwill" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_DeferredTaxAssetsOperatingLeaseLiabilities" name="DeferredTaxAssetsOperatingLeaseLiabilities" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="asmb_DeferredTaxLiabilitiesOperatingLeaseRightOfUseAssets" name="DeferredTaxLiabilitiesOperatingLeaseRightOfUseAssets" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="asmb_OperatingLossCarryForwardsExpirationPeriod" name="OperatingLossCarryForwardsExpirationPeriod" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="asmb_IncomeTaxesLineItems" name="IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="asmb_IncomeTaxesTable" name="IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
