Agency Mortgage-Backed Securities (MBS) are secured by a collection of residential mortgages or loans.. The mortgages are sold to a group of government agency that securitizes the loans together into a security that investors can buy and sell. In U.S., they may be issued by structures set up by government-sponsored enterprises like Fannie Mae or Freddie Mac. The structure of the MBS may be known as Pass-Through (PT), where the interest and principal payments from the borrower pass through it to the MBS investors, or it may be more complex, made up of a pool of other MBSs. Other types of MBS include collateralized mortgage obligations (CMO) and Structured Derivative Products (SDP).