Mortgage-backed securities represent an ownership interest in mortgage loans made by financial institutions, such as savings and loans, commercial banks or mortgage companies, to finance the borrower's purchase of a home or other real estate. The most basic mortgage securities, known as "pass-throughs," or participation certificates, represent a direct ownership interest in a trust composed of a pool of mortgage loans. The majority of mortgage securities are issued and/or guaranteed by the Government National Mortgage Association (Ginnie Mae), the Federal National Mortgage Association (Fannie Mae) and the Federal Home Loan Mortgage Corporation (Freddie Mac).