In an unusual move, Fannie Mae has obtained credit ratings for eight previously unrated tranches of five Connecticut Avenue Securities transactions (CAS) issued between 2012, when the program was launched, and 2015. CAS are a type of credit linked note that Fannie Mae uses to offload the risk of residential mortgages it insures. The notes are unsecured general obligations, but their performance is linked to that of a pool of securitized mortgages. If too many mortgages default, CAS investors could forego interest or principal. Since the program's inception, Fannie has obtained investment grade ratings for some of the less risky, M1 tranches issued in these transactions. That's because these notes appealed to investors whose investment guidelines precluded them from buying unrated debt. However the initial investors in the M2 tranches were largely indifferent to credit ratings, so it was not in Fannie's interest to pay for one.