Moody's Investors Service downgraded $650m of residential mortgage-backed securities, or RMBS, made up of Alt-A loans issued by Countrywide Financial in 2005, the latest downgrade by the ratings agency. It has downgraded many tens of billions of dollars of RMBS over the past several months as loan losses and delinquencies remain high, although signs of stabilization--and even decline--have been seen in recent data.