Moody's Investors Service downgraded ratings of 270 tranches and confirmed ratings of another four tranches within eight residential mortgage-backed securities (RMBS) transactions backed by Alt-A loans. The deals were issued by Credit Suisse First Boston (CSFB) Mortgage-Backed Pass-Through Securities in 2005. They are backed by first-lien, fixed-rate Alt-A mortgages. The downgrades arrive amid "rapidly deteriorating performance of Alt-A pools in conjunction with macroeconomic conditions that remain under duress," the credit-rating agency said Tuesday. It's part of Moody's initiative to update loss expectations on Alt-A pools issued from 2005 to 2007. The downgrades put many of the tranches further into junk status, from double-B to triple-C, for example. Moody's ran each individual pool through a number of stress scenarios to assess the rating implications of updated loss expectations on Alt-A RMBS. The scenarios include 96 different combinations within six loss levels, four timing curves and four prepayment curves. Write to Diana Golobay.