An index made up of mortgage-backed securities that is used for benchmarking purposes. The Lehman Brothers MBS Index consists of fixed-rate securities, such as mortgage pools created by the Government National Mortgage Association (GNMA), Federal Home Loan Mortgage Corp (FHLMC) and Federal National Mortgage Association (FNMA). This index serves as a performance benchmark for many mortgage-backed securities funds. 



|||For a mortgage-backed security to be included in the index, it must have a minimum principal amount of $50 million. The securities within the index have an average life between 15 and 30 years. The MBS Index is a subset of the Lehman Brothers Aggregate Bond Index, which includes a larger variety of benchmark bonds.