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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2020-03-26T19:09:53.0697300+00:00 -->
<!-- Version            : 5.0.1.369 -->
<!-- Package ID         : 34461d57b55442078f11d9e2cf5f4c24 -->
<!-- Copyright (c) 2020 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="life_DebtInstrumentNumberOfTranches" name="DebtInstrumentNumberOfTranches" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="life_SiliconValleyBankAndSolarCapitalLtdLoanAndSecurityAgreementMember" name="SiliconValleyBankAndSolarCapitalLtdLoanAndSecurityAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TermLoanAgreementTrancheOneMember" name="TermLoanAgreementTrancheOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TermLoanAgreementTrancheTwoMember" name="TermLoanAgreementTrancheTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TermLoanAgreementTrancheThreeMember" name="TermLoanAgreementTrancheThreeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_LoanAmendmentAgreementWithSVBAndSolarMember" name="LoanAmendmentAgreementWithSVBAndSolarMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_ResearchFundingAndOptionAgreementMember" name="ResearchFundingAndOptionAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TheScrippsResearchInstituteMember" name="TheScrippsResearchInstituteMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_PercentageOfFundedAmountForFinalPayment" name="PercentageOfFundedAmountForFinalPayment" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_AccretionOfFinalPaymentOfDebtInstrument" name="AccretionOfFinalPaymentOfDebtInstrument" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="life_ClassOfWarrantOrRightWarrantsExpirationPeriod" name="ClassOfWarrantOrRightWarrantsExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_FairValueOfWarrants" name="FairValueOfWarrants" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="life_TerminationOfResearchFundingAndOptionAgreementEffectiveMonthAndYear" name="TerminationOfResearchFundingAndOptionAgreementEffectiveMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_AtMarketOfferingProgramMember" name="AtMarketOfferingProgramMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_CowenCompanyLimitedLiabilityCorporationMember" name="CowenCompanyLimitedLiabilityCorporationMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_SalesAgreementMember" name="SalesAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_ATMOfferingProgramMember" name="ATMOfferingProgramMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_HCWainwrightCoLLCMember" name="HCWainwrightCoLLCMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_SecuritiesPurchaseAgreementMember" name="SecuritiesPurchaseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_VikingGlobalOpportunitiesIlliquidInvestmentsSubMasterLPMember" name="VikingGlobalOpportunitiesIlliquidInvestmentsSubMasterLPMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_RemainingPurchasersMember" name="RemainingPurchasersMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_FederatedKaufmannSmallCapFundMember" name="FederatedKaufmannSmallCapFundMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TwoThousandFourteenStockOptionPlanMember" name="TwoThousandFourteenStockOptionPlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TwoThousandFifteenStockOptionAndIncentivePlanMember" name="TwoThousandFifteenStockOptionAndIncentivePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_InducementPoolNonQualifiedOptionMember" name="InducementPoolNonQualifiedOptionMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TwoThousandFifteenEmployeeStockPurchasePlanMember" name="TwoThousandFifteenEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_CommissionRateEqualToGrossProceeds" name="CommissionRateEqualToGrossProceeds" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="life_ClassOfWarrantOrRightsWarrantExpirationDate" name="ClassOfWarrantOrRightsWarrantExpirationDate" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_GrossProceedsFromIssuanceOfPrivatePlacement" name="GrossProceedsFromIssuanceOfPrivatePlacement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="life_StockOptionMaximumTerm" name="StockOptionMaximumTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_EffectiveDateOfPlan" name="EffectiveDateOfPlan" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_AdditionalCommonStockCapitalSharesReservedForFutureIssuance" name="AdditionalCommonStockCapitalSharesReservedForFutureIssuance" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="life_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReleasedInPeriod" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReleasedInPeriod" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReleasedInPeriodWeightedAverageGrantDateFairValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsReleasedInPeriodWeightedAverageGrantDateFairValue" type="num:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_ClassOfWarrantOrRightsWarrantExpirationPeriod" name="ClassOfWarrantOrRightsWarrantExpirationPeriod" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_WarrantOneMember" name="WarrantOneMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_WarrantTwoMember" name="WarrantTwoMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_WarrantThreeMember" name="WarrantThreeMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_WarrantFourMember" name="WarrantFourMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_WarrantFiveMember" name="WarrantFiveMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_TwoThousandFifteenStockPlanMember" name="TwoThousandFifteenStockPlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_ClassXPreferredStockIfConvertedToCommonStockMember" name="ClassXPreferredStockIfConvertedToCommonStockMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_IncomeTaxReconciliationTaxCutsAndJobsActOf2017" name="IncomeTaxReconciliationTaxCutsAndJobsActOf2017" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="life_IncomeTaxLineItems" name="IncomeTaxLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_IncomeTaxTable" name="IncomeTaxTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_DeferredTaxAssetsTaxCreditCarryforwardsResearchAndOtherState" name="DeferredTaxAssetsTaxCreditCarryforwardsResearchAndOtherState" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="life_DeferredTaxAssetsLeaseLiability" name="DeferredTaxAssetsLeaseLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="life_OperatingLossCarryforwardsExpirationYear" name="OperatingLossCarryforwardsExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_DeferredTaxAssetsTaxCreditCarryforwardsResearchExpirationYear" name="DeferredTaxAssetsTaxCreditCarryforwardsResearchExpirationYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_PeriodOfChangeInOwnership" name="PeriodOfChangeInOwnership" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_PercentageOfChangeInOwnership" name="PercentageOfChangeInOwnership" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromPriorPeriodTaxPositions" name="UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromPriorPeriodTaxPositions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="life_DefinedContributionPlanApprovedMonthAndYear" name="DefinedContributionPlanApprovedMonthAndYear" type="xbrli:gYearMonthItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_DefinedContributionPlanDateOfPlanBeginning" name="DefinedContributionPlanDateOfPlanBeginning" type="xbrli:dateItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_FourHundredOnePlanMember" name="FourHundredOnePlanMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_MaximumAdditionalReceivableBasedOnAchievementOfResearchMilestones" name="MaximumAdditionalReceivableBasedOnAchievementOfResearchMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="life_PercentOwnership" name="PercentOwnership" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="life_DevelopmentRegulatoryAndSalesMilestonesMember" name="DevelopmentRegulatoryAndSalesMilestonesMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="life_StockIssuedDuringPeriodAdditionalShareOfCommonStock" name="StockIssuedDuringPeriodAdditionalShareOfCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="life_PeriodOfProject" name="PeriodOfProject" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="life_PercentageOfEstimatedProjectCostFundedByGrantor" name="PercentageOfEstimatedProjectCostFundedByGrantor" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="life_PercentageOfEstimatedProjectCostToBePaid" name="PercentageOfEstimatedProjectCostToBePaid" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
</xsd:schema>
