Fannie Mae is the largest guarantor of residential mortgage debt in the U.S., accounting for roughly $4.1T in guaranteed mortgages, or about 24% of total U.S. residential mortgage debt outstanding. Fannie Mae does not originate loans or lend directly to borrowers. Instead, Fannie Mae buys mortgage loans from banks, credit unions, and non-bank lenders, packages them into mortgage-backed securities (MBS), and sells those MBS to institutional investors. The critical feature of this model is Fannie Mae's guaranty: if a borrower defaults on a loan inside a Fannie Mae MBS trust, Fannie Mae covers the payment to the investor. This makes Fannie Mae MBS effectively credit-risk-free for investors. In exchange for absorbing this credit risk, Fannie Mae collects guaranty fees from lenders. Fannie Mae operates two segments: Single-Family (properties with four or fewer units), which represents the large majority of the guarantee book, and Multifamily (buildings with five or more units). Fannie Mae's earnings are driven by the size of its guarantee book, the level of guaranty fees it charges, and credit losses on the loans it guarantees. Home price appreciation reduces credit losses, while declining values increase them, making provisions a major source of earnings volatility. Fannie Mae has been under FHFA conservatorship since 2008, during which stockholder voting rights are suspended, and Fannie Mae builds capital solely through retained earnings.