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<!-- Generated by DFIN XBRL Schema Document - http://www.dfinsolutions.com/ - on 15-06-2026 06:29:19 AM -->
<!-- Copyright(c) 2026 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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	<import namespace="http://www.xbrl.org/dtr/type/2024-01-31" schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd"/>   
	<import namespace="http://xbrl.sec.gov/country/2025" schemaLocation="https://xbrl.sec.gov/country/2025/country-2025.xsd"/>
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	<import namespace="http://xbrl.sec.gov/oef-sr/2025" schemaLocation="https://xbrl.sec.gov/oef/2025/oef-sr-2025.xsd"/>
	<import namespace="http://fasb.org/us-gaap/2025" schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd"/>
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					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="USEquityFundsMember" order="7.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_InternationalEquityFundsMember" xlink:label="InternationalEquityFundsMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="InternationalEquityFundsMember" order="8.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/BroadBasedIndexOnly">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_BroadBasedIndexDomain" xlink:label="BroadBasedIndexDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_BloombergUSAggregateBondIndexMember" xlink:label="BloombergUSAggregateBondIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BroadBasedIndexDomain" xlink:to="BloombergUSAggregateBondIndexMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_StandardAndPoorS500StockIndexMember" xlink:label="StandardAndPoorS500StockIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BroadBasedIndexDomain" xlink:to="StandardAndPoorS500StockIndexMember" order="2.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/AdditionalIndexOnly">              
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_AdditionalIndexDomain" xlink:label="AdditionalIndexDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2065FundBlendedIndexMember" xlink:label="MFSLifetime2065FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2065FundBlendedIndexMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2060FundBlendedIndexMember" xlink:label="MFSLifetime2060FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2060FundBlendedIndexMember" order="2.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2055FundBlendedIndexMember" xlink:label="MFSLifetime2055FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2055FundBlendedIndexMember" order="3.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2045FundBlendedIndexMember" xlink:label="MFSLifetime2045FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2045FundBlendedIndexMember" order="4.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2035FundBlendedIndexMember" xlink:label="MFSLifetime2035FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2035FundBlendedIndexMember" order="5.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2025FundBlendedIndexMember" xlink:label="MFSLifetime2025FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2025FundBlendedIndexMember" order="6.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2050FundBlendedIndexMember" xlink:label="MFSLifetime2050FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2050FundBlendedIndexMember" order="7.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2030FundBlendedIndexMember" xlink:label="MFSLifetime2030FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2030FundBlendedIndexMember" order="8.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2040FundBlendedIndexMember" xlink:label="MFSLifetime2040FundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetime2040FundBlendedIndexMember" order="9.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetimeIncomeFundBlendedIndexMember" xlink:label="MFSLifetimeIncomeFundBlendedIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="MFSLifetimeIncomeFundBlendedIndexMember" order="10.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/MaturityOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_MaturityDomain" xlink:label="MaturityDomain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/IndustrySectorOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_IndustrySectorDomain" xlink:label="IndustrySectorDomain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/CreditQualityOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_CreditQualityDomain" xlink:label="CreditQualityDomain"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MoneyMarketFundsQualityMember" xlink:label="MoneyMarketFundsQualityMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="CreditQualityDomain" xlink:to="MoneyMarketFundsQualityMember" order="1.0"/>
				</link:definitionLink>
				<!--GeographicRegionOnly use only for STPR-->
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/GeographicRegionOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_GeographicRegionDomain" xlink:label="GeographicRegionDomain"/>
				</link:definitionLink>
				<link:labelLink xlink:type="extended" xlink:role="http://www.xbrl.org/2003/role/link">               
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2065FundBlendedIndexMember" xlink:label="MFSLifetime2065FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2065FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2065 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2065FundBlendedIndexMember" xlink:to="label_MFSLifetime2065FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2060FundBlendedIndexMember" xlink:label="MFSLifetime2060FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2060FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2060 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2060FundBlendedIndexMember" xlink:to="label_MFSLifetime2060FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2055FundBlendedIndexMember" xlink:label="MFSLifetime2055FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2055FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2055 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2055FundBlendedIndexMember" xlink:to="label_MFSLifetime2055FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2045FundBlendedIndexMember" xlink:label="MFSLifetime2045FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2045FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2045 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2045FundBlendedIndexMember" xlink:to="label_MFSLifetime2045FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2035FundBlendedIndexMember" xlink:label="MFSLifetime2035FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2035FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2035 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2035FundBlendedIndexMember" xlink:to="label_MFSLifetime2035FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2025FundBlendedIndexMember" xlink:label="MFSLifetime2025FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2025FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2025 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2025FundBlendedIndexMember" xlink:to="label_MFSLifetime2025FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2050FundBlendedIndexMember" xlink:label="MFSLifetime2050FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2050FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2050 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2050FundBlendedIndexMember" xlink:to="label_MFSLifetime2050FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2030FundBlendedIndexMember" xlink:label="MFSLifetime2030FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2030FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2030 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2030FundBlendedIndexMember" xlink:to="label_MFSLifetime2030FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetime2040FundBlendedIndexMember" xlink:label="MFSLifetime2040FundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetime2040FundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime 2040 Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetime2040FundBlendedIndexMember" xlink:to="label_MFSLifetime2040FundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLifetimeIncomeFundBlendedIndexMember" xlink:label="MFSLifetimeIncomeFundBlendedIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSLifetimeIncomeFundBlendedIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Lifetime Income Fund Blended Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLifetimeIncomeFundBlendedIndexMember" xlink:to="label_MFSLifetimeIncomeFundBlendedIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_BloombergUSAggregateBondIndexMember" xlink:label="BloombergUSAggregateBondIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergUSAggregateBondIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg U S Aggregate Bond Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergUSAggregateBondIndexMember" xlink:to="label_BloombergUSAggregateBondIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_StandardAndPoorS500StockIndexMember" xlink:label="StandardAndPoorS500StockIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_StandardAndPoorS500StockIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Standard And Poor s 500 Stock Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="StandardAndPoorS500StockIndexMember" xlink:to="label_StandardAndPoorS500StockIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MoneyMarketFundsQualityMember" xlink:label="MoneyMarketFundsQualityMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoneyMarketFundsQualityMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Money Market Funds Quality [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoneyMarketFundsQualityMember" xlink:to="label_MoneyMarketFundsQualityMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentDebtSecuritiesMember" xlink:label="USGovernmentDebtSecuritiesMember"/>
					<link:label xlink:type="resource" xlink:label="label_USGovernmentDebtSecuritiesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">US Government Debt Securities [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="USGovernmentDebtSecuritiesMember" xlink:to="label_USGovernmentDebtSecuritiesMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FixedIncomeSecuritiesMember" xlink:label="FixedIncomeSecuritiesMember"/>
					<link:label xlink:type="resource" xlink:label="label_FixedIncomeSecuritiesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fixed Income Securities [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FixedIncomeSecuritiesMember" xlink:to="label_FixedIncomeSecuritiesMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember" xlink:label="MoneyMarketFundsMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoneyMarketFundsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Money Market Funds [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoneyMarketFundsMember" xlink:to="label_MoneyMarketFundsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_FederalAgenciesMember" xlink:label="FederalAgenciesMember"/>
					<link:label xlink:type="resource" xlink:label="label_FederalAgenciesMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Federal Agencies [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FederalAgenciesMember" xlink:to="label_FederalAgenciesMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BondsMember" xlink:label="BondsMember"/>
					<link:label xlink:type="resource" xlink:label="label_BondsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bonds [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BondsMember" xlink:to="label_BondsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_NonTraditionalFundsMember" xlink:label="NonTraditionalFundsMember"/>
					<link:label xlink:type="resource" xlink:label="label_NonTraditionalFundsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Non Traditional Funds [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="NonTraditionalFundsMember" xlink:to="label_NonTraditionalFundsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_USEquityFundsMember" xlink:label="USEquityFundsMember"/>
					<link:label xlink:type="resource" xlink:label="label_USEquityFundsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">U S Equity Funds [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="USEquityFundsMember" xlink:to="label_USEquityFundsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_InternationalEquityFundsMember" xlink:label="InternationalEquityFundsMember"/>
					<link:label xlink:type="resource" xlink:label="label_InternationalEquityFundsMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">International Equity Funds [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="InternationalEquityFundsMember" xlink:to="label_InternationalEquityFundsMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSGlobalRealEstateFundMember" xlink:label="MFSGlobalRealEstateFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSGlobalRealEstateFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Global Real Estate Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSGlobalRealEstateFundMember" xlink:to="label_MFSGlobalRealEstateFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSMidCapValueFundMember" xlink:label="MFSMidCapValueFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSMidCapValueFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Mid Cap Value Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSMidCapValueFundMember" xlink:to="label_MFSMidCapValueFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSBlendedResearchCoreEquityFundMember" xlink:label="MFSBlendedResearchCoreEquityFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSBlendedResearchCoreEquityFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Blended Research Core Equity Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchCoreEquityFundMember" xlink:to="label_MFSBlendedResearchCoreEquityFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSResearchFundMember" xlink:label="MFSResearchFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSResearchFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Research Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSResearchFundMember" xlink:to="label_MFSResearchFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSInternationalNewDiscoveryFundMember" xlink:label="MFSInternationalNewDiscoveryFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSInternationalNewDiscoveryFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS International New Discovery Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSInternationalNewDiscoveryFundMember" xlink:to="label_MFSInternationalNewDiscoveryFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSResearchInternationalFundMember" xlink:label="MFSResearchInternationalFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSResearchInternationalFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Research International Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSResearchInternationalFundMember" xlink:to="label_MFSResearchInternationalFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSInternationalIntrinsicEquityFundMember" xlink:label="MFSInternationalIntrinsicEquityFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchMidCapEquityFundMember" xlink:to="label_MFSBlendedResearchMidCapEquityFundMember"/>
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					<link:label xlink:type="resource" xlink:label="label_MFSBlendedResearchValueEquityFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Blended Research Value Equity Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchValueEquityFundMember" xlink:to="label_MFSBlendedResearchValueEquityFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSValueFundMember" xlink:to="label_MFSValueFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSGrowthFundMember" xlink:to="label_MFSGrowthFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchGrowthEquityFundMember" xlink:to="label_MFSBlendedResearchGrowthEquityFundMember"/>
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					<link:label xlink:type="resource" xlink:label="label_MFSCommodityStrategyFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Commodity Strategy Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSCommodityStrategyFundMember" xlink:to="label_MFSCommodityStrategyFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSMidCapGrowthFundMember" xlink:to="label_MFSMidCapGrowthFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSInstitutionalMoneyMarketPortfolioMember" xlink:to="label_MFSInstitutionalMoneyMarketPortfolioMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSNewDiscoveryFundMember" xlink:to="label_MFSNewDiscoveryFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSInflationAdjustedBondFundMember" xlink:label="MFSInflationAdjustedBondFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSInflationAdjustedBondFundMember" xlink:to="label_MFSInflationAdjustedBondFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSTotalReturnBondFundMember" xlink:label="MFSTotalReturnBondFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSTotalReturnBondFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Total Return Bond Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSTotalReturnBondFundMember" xlink:to="label_MFSTotalReturnBondFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSBlendedResearchSmallCapEquityFundMember" xlink:label="MFSBlendedResearchSmallCapEquityFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSBlendedResearchSmallCapEquityFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Blended Research Small Cap Equity Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchSmallCapEquityFundMember" xlink:to="label_MFSBlendedResearchSmallCapEquityFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSInternationalGrowthFundMember" xlink:label="MFSInternationalGrowthFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSInternationalGrowthFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS International Growth Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSInternationalGrowthFundMember" xlink:to="label_MFSInternationalGrowthFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSBlendedResearchInternationalEquityFundMember" xlink:label="MFSBlendedResearchInternationalEquityFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSBlendedResearchInternationalEquityFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Blended Research International Equity Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchInternationalEquityFundMember" xlink:to="label_MFSBlendedResearchInternationalEquityFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSNewDiscoveryValueFundMember" xlink:label="MFSNewDiscoveryValueFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSNewDiscoveryValueFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS New Discovery Value Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSNewDiscoveryValueFundMember" xlink:to="label_MFSNewDiscoveryValueFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSBlendedResearchEmergingMarketsEquityFundMember" xlink:label="MFSBlendedResearchEmergingMarketsEquityFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSBlendedResearchEmergingMarketsEquityFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Blended Research Emerging Markets Equity Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSBlendedResearchEmergingMarketsEquityFundMember" xlink:to="label_MFSBlendedResearchEmergingMarketsEquityFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSEmergingMarketsEquityFundMember" xlink:label="MFSEmergingMarketsEquityFundMember"/>
					<link:label xlink:type="resource" xlink:label="label_MFSEmergingMarketsEquityFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Emerging Markets Equity Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSEmergingMarketsEquityFundMember" xlink:to="label_MFSEmergingMarketsEquityFundMember"/>
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					<link:label xlink:type="resource" xlink:label="label_MFSEmergingMarketsDebtFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Emerging Markets Debt Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSEmergingMarketsDebtFundMember" xlink:to="label_MFSEmergingMarketsDebtFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSGlobalOpportunisticBondFundMember" xlink:label="MFSGlobalOpportunisticBondFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSGlobalOpportunisticBondFundMember" xlink:to="label_MFSGlobalOpportunisticBondFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSEmergingMarketsDebtLocalCurrencyFundMember" xlink:label="MFSEmergingMarketsDebtLocalCurrencyFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSEmergingMarketsDebtLocalCurrencyFundMember" xlink:to="label_MFSEmergingMarketsDebtLocalCurrencyFundMember"/>
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					<link:label xlink:type="resource" xlink:label="label_MFSGovernmentSecuritiesFundMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">MFS Government Securities Fund [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSGovernmentSecuritiesFundMember" xlink:to="label_MFSGovernmentSecuritiesFundMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstxii_MFSLimitedMaturityFundMember" xlink:label="MFSLimitedMaturityFundMember"/>
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					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MFSLimitedMaturityFundMember" xlink:to="label_MFSLimitedMaturityFundMember"/>
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					<link:label xlink:type="resource" xlink:label="label_C000229439Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000229439 [Member]</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229439Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R4</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229442Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R6</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229441Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class A</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229440Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class C</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229443Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class I</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229438Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R1</link:label>
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										<link:label xlink:type="resource" xlink:label="label_C000229436Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R3</link:label>
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					<link:label xlink:type="resource" xlink:label="label_AssetsNet" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Net Assets</link:label>
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					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_NoTradingSymbolFlag" xlink:label="dei_NoTradingSymbolFlag"/>
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	<element name="C000119438Member" id="mstxii_C000119438Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
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	<element name="C000119434Member" id="mstxii_C000119434Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
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	<element name="C000006824Member" id="mstxii_C000006824Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
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	<element name="C000006794Member" id="mstxii_C000006794Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
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	<element name="C000006795Member" id="mstxii_C000006795Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006796Member" id="mstxii_C000006796Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006797Member" id="mstxii_C000006797Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
</schema>
