Glossary term
Agency Mortgage-Backed Security
An agency mortgage-backed security is an MBS issued or guaranteed by Ginnie Mae, Fannie Mae, or Freddie Mac and backed by pools of mortgage loans.
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What Is an Agency Mortgage-Backed Security?
An agency mortgage-backed security, or agency MBS, is a mortgage-backed security issued or guaranteed by Ginnie Mae, Fannie Mae, or Freddie Mac. The security is backed by a pool of mortgage loans, and investors receive cash flows tied to borrower principal and interest payments.
The word agency can be confusing. Ginnie Mae is a government corporation whose MBS guaranty carries the full faith and credit of the United States. Fannie Mae and Freddie Mac are government-sponsored enterprises, not federal agencies in the same legal sense, but their guaranteed MBS are commonly included in the agency MBS market.
Key Takeaways
- Agency MBS are mortgage-backed securities issued or guaranteed by Ginnie Mae, Fannie Mae, or Freddie Mac.
- They are backed by pools of residential or multifamily mortgage loans.
- Investors face interest-rate and prepayment risk even when credit risk is reduced by a guaranty.
- Ginnie Mae MBS have an explicit full-faith-and-credit U.S. government guaranty.
- Fannie Mae and Freddie Mac securities are associated with GSE guarantees and FHFA conservatorship, but they are not the same as Treasury securities.
How Agency MBS Work
Mortgage lenders originate loans and sell eligible loans into the secondary market. The loans are pooled, securitized, and turned into securities that pass through principal and interest payments to investors, after fees and servicing arrangements.
The agency or GSE guaranty changes the credit-risk profile. Investors are generally not underwriting each borrower as if they directly owned the loan. Instead, they focus heavily on the guarantor, the mortgage pool characteristics, interest rates, prepayment behavior, and market liquidity.
Prepayment and Interest-Rate Risk
Agency MBS can still be risky. When mortgage rates fall, homeowners may refinance, causing investors to receive principal back sooner than expected and reinvest at lower yields. When rates rise, refinancing slows and the investor may be stuck with a lower-yielding security for longer than expected.
This extension-and-contraction risk makes agency MBS different from ordinary bonds. Cash flows depend not only on scheduled payments but also on borrower behavior, housing turnover, refinance incentives, and mortgage-rate volatility.
Agency MBS Versus Private-Label MBS
Type | Guaranty | Main investor focus |
|---|---|---|
Agency MBS | Ginnie Mae, Fannie Mae, or Freddie Mac guaranty | Rates, prepayments, guarantor framework, liquidity |
Private-label MBS | No agency or GSE guaranty | Credit quality, collateral, structure, subordination, servicer performance |
Agency MBS usually have lower credit risk than private-label MBS, but lower credit risk does not mean no risk. Duration, convexity, and prepayment assumptions can still create meaningful gains or losses.
Why Investors Own Them
Agency MBS are large, liquid fixed-income instruments used by banks, insurers, pension funds, mutual funds, mortgage REITs, and the Federal Reserve. They can provide yield above Treasuries, exposure to U.S. housing finance, and portfolio diversification.
The tradeoff is complexity. A simple yield quote can hide assumptions about prepayments and interest-rate paths. Investors should look at duration, convexity, option-adjusted spread, pool characteristics, and the guarantor.
The Bottom Line
An agency mortgage-backed security is a major fixed-income instrument backed by mortgage pools and an agency or GSE guaranty. It can reduce credit-risk concerns, but it leaves investors with important interest-rate, prepayment, liquidity, and structural risks.