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<!-- Generated by DFIN XBRL Schema Document - http://www.dfinsolutions.com/ - on 12-06-2026 03:14:36 PM -->
<!-- Copyright(c) 2026 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
<schema targetNamespace="http://www.funds.mfs/20260609" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:mstix="http://www.funds.mfs/20260609" xmlns:dei="http://xbrl.sec.gov/dei/2025" xmlns:srt-types="http://fasb.org/srt-types/2025" xmlns:deprecated="http://www.xbrl.org/2009/role/deprecated" xmlns:dtr="http://www.xbrl.org/2009/dtr" xmlns:oef="http://xbrl.sec.gov/oef/2025" xmlns:ref="http://www.xbrl.org/2006/ref" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xl="http://www.xbrl.org/2003/XLink" xmlns:us-gaap="http://fasb.org/us-gaap/2025" xmlns:oef-pre="http://xbrl.sec.gov/oef-pre/2025" xmlns:oef-lab="http://xbrl.sec.gov/oef-lab/2025" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:country="http://xbrl.sec.gov/country/2025" xmlns:us-roles="http://fasb.org/us-roles/2025" xmlns:enum2="http://xbrl.org/2020/extensible-enumerations-2.0" xmlns:us-types="http://fasb.org/us-types/2025" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:oef-def="http://xbrl.sec.gov/oef-def/2025" xmlns:negated="http://www.xbrl.org/2009/role/negated" xmlns:xbrldi="http://xbrl.org/2006/xbrldi" xmlns:srt="http://fasb.org/srt/2025" xmlns:oef-sr="http://xbrl.sec.gov/oef-sr/2025" xmlns:srt-roles="http://fasb.org/srt-roles/2025" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:stpr="http://xbrl.sec.gov/stpr/2025" elementFormDefault="qualified">
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	<import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
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	<import namespace="http://fasb.org/us-types/2025" schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-types-2025.xsd"/>
	<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"/>
	<import namespace="http://fasb.org/srt/2025" schemaLocation="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd"/>
	<import namespace="http://fasb.org/srt-types/2025" schemaLocation="https://xbrl.fasb.org/srt/2025/elts/srt-types-2025.xsd"/>
	<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"/>
	<import namespace="http://fasb.org/us-roles/2025" schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd"/>
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				<link:roleRef roleURI="http://xbrl.sec.gov/oef/role/ShareholderReportHoldingsDetails" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#ShareholderReportHoldingsDetails"/>
				<link:roleRef roleURI="http://xbrl.sec.gov/oef/role/ShareholderReportLineGraphDetails" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#ShareholderReportLineGraphDetails"/>
				<link:roleRef roleURI="http://xbrl.sec.gov/oef/role/ShareholderReportAverageAnnualReturnDetails" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#ShareholderReportAverageAnnualReturnDetails"/>
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				<link:roleRef roleURI="http://xbrl.sec.gov/oef/role/LargestHoldingOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#LargestHoldingOnly"/>
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				<link:roleRef roleURI="http://xbrl.sec.gov/oef/role/BroadBasedIndexOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#BroadBasedIndexOnly"/>
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					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006718Member" order="19.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006720Member" xlink:label="C000006720Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006720Member" order="20.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006722Member" xlink:label="C000006722Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006722Member" order="21.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006723Member" xlink:label="C000006723Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006723Member" order="22.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006712Member" xlink:label="C000006712Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006712Member" order="23.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000070424Member" xlink:label="C000070424Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000070424Member" order="24.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006745Member" xlink:label="C000006745Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006745Member" order="25.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006750Member" xlink:label="C000006750Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006750Member" order="26.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006751Member" xlink:label="C000006751Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006751Member" order="27.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006744Member" xlink:label="C000006744Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006744Member" order="28.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006753Member" xlink:label="C000006753Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006753Member" order="29.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006755Member" xlink:label="C000006755Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006755Member" order="30.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006756Member" xlink:label="C000006756Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006756Member" order="31.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006746Member" xlink:label="C000006746Member"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="ShareClassDomain" xlink:to="C000006746Member" order="32.0"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/Other1CategoryOnly">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_OtherCategory1Domain" xlink:label="OtherCategory1Domain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/Other2CategoryOnly">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_OtherCategory2Domain" xlink:label="OtherCategory2Domain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/LargestHoldingOnly">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_LargestHoldingsDomain" xlink:label="LargestHoldingsDomain"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/oef/role/SalesLoadOnly">
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/oef/2025/oef-2025.xsd#oef_WithSalesLoadMember" xlink:label="WithSalesLoadMember"/>
				</link:definitionLink>
				<link:definitionLink xlink:type="extended" xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists">               
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="InvestmentTypeCategorizationMember"/>
					<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:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="USGovernmentDebtSecuritiesMember" order="1.0"/>
					<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:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="MoneyMarketFundsMember" order="2.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_FederalAgenciesMember" xlink:label="FederalAgenciesMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="FederalAgenciesMember" order="3.0"/>
					<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:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="InvestmentTypeCategorizationMember" xlink:to="FixedIncomeSecuritiesMember" order="4.0"/>
				</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="#mstix_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="#mstix_BloombergMunicipalBondIndexMember" xlink:label="BloombergMunicipalBondIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="BroadBasedIndexDomain" xlink:to="BloombergMunicipalBondIndexMember" 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="#mstix_Bloomberg13YearUSGovernmentCreditBondIndexMember" xlink:label="Bloomberg13YearUSGovernmentCreditBondIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="Bloomberg13YearUSGovernmentCreditBondIndexMember" order="1.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_Bloomberg19YearMunicipalBondIndexMember" xlink:label="Bloomberg19YearMunicipalBondIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="Bloomberg19YearMunicipalBondIndexMember" order="2.0"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_BloombergUSCreditIndexMember" xlink:label="BloombergUSCreditIndexMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="AdditionalIndexDomain" xlink:to="BloombergUSCreditIndexMember" order="3.0"/>
				</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="#mstix_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:loc xlink:type="locator" xlink:href="#mstix_NotRatedMember" xlink:label="NotRatedMember"/>
					<link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="CreditQualityDomain" xlink:to="NotRatedMember" order="2.0"/>
				</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="#mstix_Bloomberg13YearUSGovernmentCreditBondIndexMember" xlink:label="Bloomberg13YearUSGovernmentCreditBondIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_Bloomberg13YearUSGovernmentCreditBondIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg 1 3 Year U S Government Credit Bond Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="Bloomberg13YearUSGovernmentCreditBondIndexMember" xlink:to="label_Bloomberg13YearUSGovernmentCreditBondIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_Bloomberg19YearMunicipalBondIndexMember" xlink:label="Bloomberg19YearMunicipalBondIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_Bloomberg19YearMunicipalBondIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg 1 9 Year Municipal Bond Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="Bloomberg19YearMunicipalBondIndexMember" xlink:to="label_Bloomberg19YearMunicipalBondIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_BloombergUSCreditIndexMember" xlink:label="BloombergUSCreditIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergUSCreditIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg U S Credit Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergUSCreditIndexMember" xlink:to="label_BloombergUSCreditIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_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="#mstix_BloombergMunicipalBondIndexMember" xlink:label="BloombergMunicipalBondIndexMember"/>
					<link:label xlink:type="resource" xlink:label="label_BloombergMunicipalBondIndexMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Bloomberg Municipal Bond Index [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="BloombergMunicipalBondIndexMember" xlink:to="label_BloombergMunicipalBondIndexMember"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_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="#mstix_NotRatedMember" xlink:label="NotRatedMember"/>
					<link:label xlink:type="resource" xlink:label="label_NotRatedMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Not Rated [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="NotRatedMember" xlink:to="label_NotRatedMember"/>
					<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_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="#mstix_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_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="#mstix_C000006757Member" xlink:label="C000006757Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006757Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006757 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006757Member" xlink:to="label_C000006757Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006757Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class A</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006757Member" xlink:to="label_C000006757Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006758Member" xlink:label="C000006758Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006758Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006758 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006758Member" xlink:to="label_C000006758Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006758Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class B</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006758Member" xlink:to="label_C000006758Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006759Member" xlink:label="C000006759Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006759Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006759 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006759Member" xlink:to="label_C000006759Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006759Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class C</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006759Member" xlink:to="label_C000006759Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000092113Member" xlink:label="C000092113Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000092113Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000092113 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000092113Member" xlink:to="label_C000092113Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000092113Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class I</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000092113Member" xlink:to="label_C000092113Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000195152Member" xlink:label="C000195152Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000195152Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000195152 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000195152Member" xlink:to="label_C000195152Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000195152Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R6</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000195152Member" xlink:to="label_C000195152Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006760Member" xlink:label="C000006760Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006760Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006760 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006760Member" xlink:to="label_C000006760Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006765Member" xlink:label="C000006765Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006765Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006765 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006765Member" xlink:to="label_C000006765Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006766Member" xlink:label="C000006766Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006766Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006766 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006766Member" xlink:to="label_C000006766Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006767Member" xlink:label="C000006767Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006767Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006767 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006767Member" xlink:to="label_C000006767Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006769Member" xlink:label="C000006769Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006769Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006769 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006769Member" xlink:to="label_C000006769Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006769Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R1</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006769Member" xlink:to="label_C000006769Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006771Member" xlink:label="C000006771Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006771Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006771 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006771Member" xlink:to="label_C000006771Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006771Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R2</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006771Member" xlink:to="label_C000006771Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006772Member" xlink:label="C000006772Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006772Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006772 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006772Member" xlink:to="label_C000006772Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006772Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R3</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006772Member" xlink:to="label_C000006772Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006761Member" xlink:label="C000006761Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006761Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006761 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006761Member" xlink:to="label_C000006761Member"/>
										<link:label xlink:type="resource" xlink:label="label_C000006761Member_2" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xml:lang="en-US">Class R4</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006761Member" xlink:to="label_C000006761Member_2"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000034443Member" xlink:label="C000034443Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000034443Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000034443 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000034443Member" xlink:to="label_C000034443Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006716Member" xlink:label="C000006716Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006716Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006716 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006716Member" xlink:to="label_C000006716Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006711Member" xlink:label="C000006711Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006711Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006711 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006711Member" xlink:to="label_C000006711Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000119776Member" xlink:label="C000119776Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000119776Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000119776 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000119776Member" xlink:to="label_C000119776Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006717Member" xlink:label="C000006717Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006717Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006717 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006717Member" xlink:to="label_C000006717Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006718Member" xlink:label="C000006718Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006718Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006718 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006718Member" xlink:to="label_C000006718Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006720Member" xlink:label="C000006720Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006720Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006720 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006720Member" xlink:to="label_C000006720Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006722Member" xlink:label="C000006722Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006722Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006722 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006722Member" xlink:to="label_C000006722Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006723Member" xlink:label="C000006723Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006723Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006723 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006723Member" xlink:to="label_C000006723Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006712Member" xlink:label="C000006712Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006712Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006712 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006712Member" xlink:to="label_C000006712Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000070424Member" xlink:label="C000070424Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000070424Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000070424 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000070424Member" xlink:to="label_C000070424Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006745Member" xlink:label="C000006745Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006745Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006745 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006745Member" xlink:to="label_C000006745Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006750Member" xlink:label="C000006750Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006750Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006750 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006750Member" xlink:to="label_C000006750Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006751Member" xlink:label="C000006751Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006751Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006751 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006751Member" xlink:to="label_C000006751Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006744Member" xlink:label="C000006744Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006744Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006744 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006744Member" xlink:to="label_C000006744Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006753Member" xlink:label="C000006753Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006753Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006753 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006753Member" xlink:to="label_C000006753Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006755Member" xlink:label="C000006755Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006755Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006755 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006755Member" xlink:to="label_C000006755Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006756Member" xlink:label="C000006756Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006756Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006756 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006756Member" xlink:to="label_C000006756Member"/>
					<link:loc xlink:type="locator" xlink:href="#mstix_C000006746Member" xlink:label="C000006746Member"/>
					<link:label xlink:type="resource" xlink:label="label_C000006746Member" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">C000006746 [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="C000006746Member" xlink:to="label_C000006746Member"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MoodysAaaRatingMember" xlink:label="MoodysAaaRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_MoodysAaaRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Moody&apos;s, Aaa Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="MoodysAaaRatingMember" xlink:to="label_MoodysAaaRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchAARatingMember" xlink:label="FitchAARatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchAARatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, AA Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchAARatingMember" xlink:to="label_FitchAARatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchBBBRatingMember" xlink:label="FitchBBBRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchBBBRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, BBB Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchBBBRatingMember" xlink:to="label_FitchBBBRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchARatingMember" xlink:label="FitchARatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchARatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, A Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchARatingMember" xlink:to="label_FitchARatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchBBRatingMember" xlink:label="FitchBBRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchBBRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, BB Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchBBRatingMember" xlink:to="label_FitchBBRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchBRatingMember" xlink:label="FitchBRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchBRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, B Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchBRatingMember" xlink:to="label_FitchBRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchDRatingMember" xlink:label="FitchDRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchDRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, D Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchDRatingMember" xlink:to="label_FitchDRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchCCCRatingMember" xlink:label="FitchCCCRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchCCCRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, CCC Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchCCCRatingMember" xlink:to="label_FitchCCCRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_FitchCCRatingMember" xlink:label="FitchCCRatingMember"/>
					<link:label xlink:type="resource" xlink:label="label_FitchCCRatingMember" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Fitch, CC Rating [Member]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="FitchCCRatingMember" xlink:to="label_FitchCCRatingMember"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentTypeAxis" xlink:label="InvestmentTypeAxis"/>
					<link:label xlink:type="resource" xlink:label="label_InvestmentTypeAxis" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Investment Type [Axis]</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="InvestmentTypeAxis" xlink:to="label_InvestmentTypeAxis" priority="1"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsNet" xlink:label="AssetsNet"/>
					<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>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="AssetsNet" xlink:to="label_AssetsNet" priority="1"/>
					<link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentCompanyPortfolioTurnover" xlink:label="InvestmentCompanyPortfolioTurnover"/>
					<link:label xlink:type="resource" xlink:label="label_InvestmentCompanyPortfolioTurnover" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">Investment Company Portfolio Turnover</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="InvestmentCompanyPortfolioTurnover" xlink:to="label_InvestmentCompanyPortfolioTurnover" priority="1"/>                
					<link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_NoTradingSymbolFlag" xlink:label="dei_NoTradingSymbolFlag"/>
					<link:label xlink:type="resource" xlink:label="label_NoTradingSymbolFlag" xlink:role="http://www.xbrl.org/2003/role/label" xml:lang="en-US">No Trading Symbol Flag</link:label>
					<link:labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_NoTradingSymbolFlag" xlink:to="label_NoTradingSymbolFlag" priority="1"/>		
				</link:labelLink>
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	</annotation>   
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	<element name="Bloomberg19YearMunicipalBondIndexMember" id="mstix_Bloomberg19YearMunicipalBondIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergUSCreditIndexMember" id="mstix_BloombergUSCreditIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergUSAggregateBondIndexMember" id="mstix_BloombergUSAggregateBondIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="BloombergMunicipalBondIndexMember" id="mstix_BloombergMunicipalBondIndexMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="MoneyMarketFundsQualityMember" id="mstix_MoneyMarketFundsQualityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="NotRatedMember" id="mstix_NotRatedMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="FederalAgenciesMember" id="mstix_FederalAgenciesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006757Member" id="mstix_C000006757Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006758Member" id="mstix_C000006758Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
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	<element name="C000006756Member" id="mstix_C000006756Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
	<element name="C000006746Member" id="mstix_C000006746Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" nillable="true" xbrli:periodType="duration"/>
</schema>
