MBS stands for mortgage-backed security, an investment created by bundling hundreds or thousands of home loans into a single asset that trades on financial markets. Each security earns its return from the monthly mortgage payments made by the borrowers in the pool, giving investors exposure to the housing market without owning property or lending to homeowners directly. As of December 2025, the outstanding balance of single-family agency MBS alone exceeded $9.2 trillion, split among Ginnie Mae, Fannie Mae, and Freddie Mac.1Ginnie Mae. Global Markets Analysis Report
How an MBS Is Built From Individual Home Loans
It starts with an ordinary mortgage. A bank or mortgage lender funds a home purchase, the borrower signs a promissory note agreeing to repay, and the property serves as collateral. Rather than hold that loan for its full 15 or 30 years, the lender usually sells it into the secondary market. That sale frees up capital so the lender can write new mortgages.
The buyer collects many such loans and groups them into a pool based on shared characteristics: similar interest rates, similar terms, similar borrower credit profiles. Consistency makes the pool’s behavior more predictable.
To keep the pooled loans legally separate from the original lender’s finances, the buyer sets up a Special Purpose Vehicle (SPV). This is an independent legal entity that takes title to the mortgages. If the lender later goes bankrupt, its creditors can’t reach the loans inside the SPV. The security’s value depends only on how the borrowers in the pool perform.
The SPV then issues certificates representing fractional ownership of the pool, and those certificates are sold to investors. Buyers of the certificates have a legal right to the income the loans generate. The pipeline runs in a loop: money from investors flows back into neighborhoods as new mortgage funding.
Who Issues Mortgage-Backed Securities
Three groups issue almost all MBS, and the identity of the issuer determines the guarantee behind the security and the kinds of loans in the pool.
Ginnie Mae
The Government National Mortgage Association, or Ginnie Mae, is a government corporation within the Department of Housing and Urban Development.2Federal Register. Agencies – Government National Mortgage Association Ginnie Mae doesn’t buy or sell loans itself. It guarantees timely payment of principal and interest on securities backed by federally insured or guaranteed loans, mainly those from the Federal Housing Administration (FHA) and the Department of Veterans Affairs (VA).3Ginnie Mae. Funding Government Lending Ginnie Mae securities are the only MBS backed by the full faith and credit of the United States government, so the federal government stands behind payments even if an issuer or borrowers default.4Office of the Law Revision Counsel. 12 USC 1721 – Management and Liquidation Functions of Government National Mortgage Association
Fannie Mae and Freddie Mac
Fannie Mae and Freddie Mac are government-sponsored enterprises (GSEs) that buy qualifying mortgages from lenders and package them into what the market calls “agency MBS.” Fannie Mae operates under the Federal National Mortgage Association Charter Act; Freddie Mac is governed by the Federal Home Loan Mortgage Corporation Act.5Office of the Law Revision Counsel. 12 USC 1716 – Declaration of Purposes of Subchapter6Office of the Law Revision Counsel. 12 USC 1451 – Definitions Both guarantee timely payment of principal and interest on their securities, but the guarantee comes from the agencies themselves, not the U.S. Treasury. The federal government placed both into conservatorship in 2008, yet their MBS still do not carry the same explicit full-faith-and-credit backing as Ginnie Mae securities.
To qualify for purchase by Fannie or Freddie, a mortgage must fall within the conforming loan limit set each year by the Federal Housing Finance Agency. For 2026 the baseline limit for a single-family home in most of the country is $832,750, up $26,250 from 2025. In designated high-cost areas the ceiling rises to $1,249,125, or 150 percent of the baseline.7FHFA. FHFA Announces Conforming Loan Limit Values for 2026 Loans above these limits are called jumbo mortgages and can’t be securitized through Fannie Mae or Freddie Mac.
Private-Label Issuers
Large investment banks and other private financial firms issue their own MBS, generally called “private-label” securities. They carry no government or GSE guarantee, and they often include loans that sit outside the underwriting standards Fannie and Freddie require: jumbo mortgages, loans to borrowers with lower credit scores, or unconventional terms. Because of the added risk, private-label MBS typically offer higher yields to attract investors.
Private-label issuers must register their securities under the Securities Act of 1933 and the Securities Exchange Act of 1934.8Federal Register. Concept Release on Residential Mortgage-Backed Securities Disclosures and Enhancements to Asset-Backed Securities Registration Under Regulation AB they must provide asset-level data for each loan in the pool, including the original amount, interest rate, borrower credit score, loan-to-value ratio, occupancy status, and property type.9eCFR. Subpart 229.1100 Asset-Backed Securities (Regulation AB) Agency MBS issued through Ginnie Mae, Fannie Mae, and Freddie Mac are exempt from these SEC registration and reporting requirements.
The Main Types of MBS
Pass-Through Securities
The simplest MBS is the pass-through. Borrower payments flow directly to investors on a proportional basis. Own ten percent of a pass-through, and you receive ten percent of the payments collected from the pool after servicing fees. Every investor shares the same exposure to prepayments, defaults, and interest-rate moves.
Collateralized Mortgage Obligations
A collateralized mortgage obligation (CMO) starts with the same pool of mortgages but carves the cash flows into slices called tranches. Each tranche has its own rules about when it receives interest and principal. Some absorb all prepayments first and pay off quickly; others receive no principal until the earlier tranches are fully retired. That hierarchy lets investors pick a tranche that matches their time horizon.
Because CMOs redistribute cash flows rather than passing them through evenly, risk varies sharply across tranches. Senior tranches get paid first and carry less default risk. Junior tranches absorb losses first but offer higher yields. Rating agencies evaluate each tranche separately, weighing borrower credit scores, property values, and the equity cushion in the underlying loans.
Commercial Mortgage-Backed Securities
Commercial mortgage-backed securities (CMBS) are backed by loans on income-producing commercial properties: office buildings, retail centers, hotels, apartment complexes. CMBS pools are far smaller, typically containing between 40 and 250 loans versus the thousands in residential pools.10Federal Reserve Board. Differences Across Originators in CMBS Loan Underwriting Each loan represents a much larger share of the pool, so the credit quality of individual loans matters more in CMBS pricing than in residential MBS, where the math relies on statistical patterns across thousands of borrowers.
How the Cash Reaches Investors
When a homeowner sends in a monthly mortgage payment, it goes first to a mortgage servicer. The servicer collects payments, manages escrow for taxes and insurance, and handles delinquencies. It deducts a fee before passing the rest to investors. For Fannie Mae fixed-rate loans the maximum servicing fee is 50 basis points (0.50 percent) of the outstanding loan balance annually.11Fannie Mae. C2-1.1-05, Servicing Fees Fees for other loan types generally fall within a similar range.
Each payment an investor receives has two parts: interest earned on the outstanding debt, and a portion of the original principal being repaid. Unlike a corporate bond that pays a fixed coupon until maturity, MBS payments move month to month. When borrowers refinance, sell their homes, or make extra payments, principal comes back faster than scheduled. When borrowers stay put and pay on time, principal returns slowly over the full life of the loan. Your income stream tracks the real behavior of the borrowers in the pool.
What Makes MBS Riskier Than a Standard Bond
Two risks set MBS apart from other fixed-income investments, and both trace back to the same source: a homeowner’s ability to pay off a mortgage ahead of schedule.
Prepayment Risk
When interest rates fall, many borrowers refinance at the new lower rate and pay off the original loan early. For MBS investors, principal returns sooner than expected, arriving exactly when reinvestment yields are lower. A Federal Reserve Bank of Kansas City analysis showed the effect clearly: a one-percentage-point drop in rates on a pool of 30-year mortgages would theoretically produce a 6.1 percent capital gain if prepayments stayed normal, but refinancing-driven prepayments cut that gain to roughly 3.8 percent.12Federal Reserve Bank of Kansas City. The Prepayment Risk of Mortgage-Backed Securities
Extension Risk
The opposite happens when rates rise. Fewer borrowers refinance, prepayments slow, and investors hold a below-market security for longer than planned. MBS prices tend to fall faster than comparable Treasury prices in a rising-rate environment because the expected duration of the security keeps stretching.13Liberty Street Economics. Convexity Event Risks in a Rising Interest Rate Environment This feature is called negative convexity: you can lose more than expected when rates climb and gain less than expected when they fall.
Credit Risk
Credit risk (the chance borrowers stop paying altogether) varies sharply by issuer. Ginnie Mae carries a federal guarantee, and Fannie Mae and Freddie Mac absorb credit losses through their own guarantees, so credit risk to the investor is minimal for agency MBS. Private-label MBS pass default losses through to investors, with junior tranches taking the first hit. Buyers of private-label securities have to look closely at underwriting quality and borrower profiles in the pool.
How an Individual Can Buy Into the MBS Market
You can buy individual agency MBS through a brokerage account. New Ginnie Mae securities require a minimum of $25,000, with additional purchases in $1,000 increments. Fannie Mae and Freddie Mac securities are available in $1,000 increments, which puts them within reach of smaller investors.
For most people, mutual funds and ETFs that focus on mortgage-backed securities are a simpler way in. These funds hold diversified pools of MBS and take on the work of tracking prepayments, analyzing tranches, and reinvesting returned principal. Funds that emphasize agency MBS generally sit within the “intermediate government” investment category.