Fannie Mae and Freddie Mac (the Enterprises) operate under congressional charters to provide liquidity, stability, and affordability to the mortgage market. Those charters, which have been amended from time to time, authorize the Enterprises to purchase residential mortgages and codify an affirmative obligation to facilitate the financing of affordable housing for low and moderate-income families. Pursuant to their charters, the Enterprises may purchase single-family residential mortgages with loan-to-value (LTV) ratios above 80%, provided that these mortgages are supported by one of several credit enhancements identified in their charters. A credit enhancement is a method or tool to reduce the risk of extending credit to a borrower; mortgage insurance is one such method.