TL;DR. A mortgage-backed security (MBS) is a bond whose cash flows come from a pool of home loans. Borrowers pay principal and interest each month; servicers collect those payments, take a small fee, and pass the rest through to MBS investors. Agency MBS — issued by Ginnie Mae, Fannie Mae, and Freddie Mac — dominate the market, and the Federal Reserve currently holds about $1.97 trillion of them on its balance sheet. The hardest concept is not the structure but the risk: when rates fall, borrowers refinance early and your bond shortens; when rates rise, refis stop and your bond lengthens. That asymmetric duration is called negative convexity, and it is why MBS yields trade well above Treasuries.
What is an MBS, in one sentence?
The Securities and Exchange Commission’s plain-language definition is the cleanest place to start. MBS are “debt obligations that represent claims to the cash flows from pools of mortgage loans, most commonly on residential property” (SEC Investor.gov). Strip away the legal language and an MBS is just a bond whose monthly payments come from a few thousand homeowners instead of a single company or government.
That seemingly small swap — one borrower to thousands — changes everything about how the bond behaves, and is why MBS need their own chapter in any fixed-income textbook.
The three pieces: borrower, servicer, security
Every MBS rides on three roles:
- The borrower takes out a 15- or 30-year fixed mortgage from a lender. The 30-year fixed-rate mortgage averaged 6.53% in the week ended May 28, 2026 (Freddie Mac PMMS via FRED).
- The servicer collects the monthly payment, keeps a small slice (typically 25–50 basis points) as the servicing fee, and forwards the rest.
- The MBS investor — a pension fund, bank, REIT, mutual fund, or central bank — receives a pro-rata share of the principal and interest from the pool.
In between sits the issuer, who buys the loans from the originating lender, pools them, and sells the bonds. In the U.S., that issuer is almost always Ginnie Mae, Fannie Mae, or Freddie Mac.
Pass-throughs: the original MBS structure
The simplest MBS is a pass-through certificate. The SEC describes it well: pass-throughs “entitle the holder to a pro-rata share of all principal and interest payments made on the pool of mortgage loans” (SEC Investor.gov). If you own 1% of a pool, you receive 1% of every dollar the pool produces — scheduled principal, interest, and any prepayments — minus the servicing and guaranty fees.
The analogy: a pass-through is to a single bond what an ETF is to a single stock. You own a sliver of the pool; the structure simply redistributes the cash that flows in. There is no waterfall, no senior/junior split, no synthetic exposure — just pro-rata.
Agency vs non-agency: who actually guarantees the bond
The “agency” in agency MBS refers to the issuer. The three agencies are not equal, and the difference matters for credit risk.
| Issuer | What it guarantees | Credit backing | Underlying loans |
|---|---|---|---|
| Ginnie Mae | Timely P&I to investors | Full faith and credit of the U.S. government | FHA, VA, USDA, HUD PIH |
| Fannie Mae | Timely P&I to investors | Corporate guaranty (Fannie Mae); under FHFA conservatorship since 2008 | Conventional conforming (single-family up to $806,500 in 2025) |
| Freddie Mac | Timely P&I to investors | Corporate guaranty (Freddie Mac); under FHFA conservatorship since 2008 | Conventional conforming (single-family up to $806,500 in 2025) |
| Non-agency (“private label”) | No agency guaranty | Senior/subordinate credit enhancement only | Jumbo, non-QM, investor loans |
Ginnie Mae is unique. As its own materials state, it is “the only MBS to carry the explicit full faith and credit of the United States government” (Ginnie Mae). Fannie and Freddie MBS carry only a corporate guaranty; the market treats it as nearly equivalent to a Treasury guaranty because both GSEs have been under FHFA conservatorship since September 2008, but the legal distinction is real and matters in stress scenarios.
Non-agency MBS — the so-called private-label securities (PLS) that headlined the 2008 crisis — have no agency wrap. Investors rely on senior/subordinate tranching and excess spread for credit protection. PLS issuance collapsed after 2008 and remains a small fraction of the market today.
CMOs: slicing a pass-through into tranches
A pass-through gives every investor the same cash-flow timing. That is a problem for institutions that want short bonds and others that want long bonds from the same pool. The fix was the collateralised mortgage obligation, or CMO.
The SEC’s definition: CMOs are “multiple classes of securities designed to appeal to investors with different investment objectives and risk tolerances”, where “principal and interest payments made on the pool of mortgage loans are distributed to the different classes of securities, known as ‘tranches’, according to a priority of payments” (SEC Investor.gov).
The simplest CMO is a sequential-pay structure. All principal — scheduled and prepaid — goes to Tranche A first until A is fully paid down. Then to B. Then to C. Each tranche gets a different effective maturity from the same underlying pool. More complex CMOs add planned amortization classes (PACs), targeted amortization classes (TACs), companions, interest-only (IO) strips, principal-only (PO) strips, inverse floaters, and Z-bonds. Each is a different way of carving up the same prepayment risk.
Important: CMOs do not eliminate prepayment risk. They just hand it to whichever tranche is structured to absorb it.
The risk that defines MBS: prepayment and negative convexity
A homeowner can prepay her mortgage at any time, for any reason, without penalty on a standard agency loan. She might sell the house, refinance into a lower rate, or come into cash. From the investor’s seat, that prepayment optionality is a one-way street:
- Rates fall → refinances surge → the MBS shortens. The investor gets her principal back early, at par, and now has to reinvest it at the new, lower market rate. That is called contraction risk.
- Rates rise → refis dry up → the MBS lengthens. The investor is stuck holding a low-coupon bond longer than expected, and could have earned more elsewhere. That is extension risk.
This asymmetry — the bond shortens when you don’t want it to and lengthens when you don’t want it to — is called negative convexity. For a primer on convexity in plain-vanilla bonds, see our explainer on bond duration and convexity. The compensation MBS investors demand for living with negative convexity is the option-adjusted spread (OAS) over comparable-duration Treasuries — usually a few dozen basis points.
Where the MBS market sits right now
The simplest way to read the current MBS landscape is to put the primary mortgage rate next to the 10-year Treasury — the bond MBS yields are most often quoted against — and look at the spread, then layer in what the Fed has been doing to its holdings.
The primary-mortgage-to-Treasury spread of about 205 basis points is wider than the long-run average and reflects the option-cost compensation MBS investors are still demanding in a higher-volatility regime. When the Fed was an aggressive net buyer of agency MBS during pandemic-era QE, the spread compressed sharply; as the Fed has rolled MBS off its balance sheet, the spread has stayed wide.
The Fed’s MBS holdings tell that story directly. As of the H.4.1 release for the week ended May 27, 2026, the Fed held $1,964,786 million — about $1.97 trillion — of agency MBS, down from $2,156,161 million a year earlier (Federal Reserve H.4.1; series WSHOMCB). That’s roughly $191 billion of passive runoff in twelve months, all driven by prepayments and scheduled paydowns rather than active sales.
Common mistakes
- Treating Fannie/Freddie like Treasuries. The market does, but the legal guaranty is corporate, not sovereign. Ginnie Mae is the only MBS with explicit full faith and credit.
- Confusing the note rate with the MBS coupon. Borrowers paid 6.53% on a new May 28 loan, but the corresponding MBS coupon will be roughly 50–60 bps lower after servicing and guaranty fees come out.
- Forgetting prepayment optionality. Modeling an MBS like a corporate bond with a fixed schedule will miss the cash-flow surprises that drive realised returns.
- Assuming CMOs reduce risk. They redistribute it. A “stable” PAC tranche only stays stable inside its prepayment band; outside that band it behaves like its companion.
What to read next
- Bond Duration & Convexity: Why Long Bonds Move Most — the building block under negative convexity.
- CLOs Explained — the tranched-structured-credit cousin of CMOs.
- QE vs QT: How the Fed Balance Sheet Actually Works — why the Fed’s MBS runoff matters for mortgage spreads.
Sources
- SEC Investor.gov — Mortgage-Backed Securities and Collateralized Mortgage Obligations
- Ginnie Mae — Our Mission
- Fannie Mae — Who We Are
- FHFA — 2025 Conforming Loan Limit Values
- FRED — 30-Year Fixed Rate Mortgage Average (MORTGAGE30US)
- FRED — 10-Year Treasury Constant Maturity (DGS10)
- FRED — Fed Agency MBS Holdings (WSHOMCB)
- Federal Reserve H.4.1 Statistical Release
- FHFA — Single-Family Credit Guarantee Fee Report
Disclosure: This article was produced with AI assistance and reviewed before publication. It is for informational purposes only and is not investment advice.