According to a new report issued by the agency, the October actions bring CDO downgrades for calendar year 2007 to roughly US$13 billion in 338 SF CDO tranches from 173 deals. The majority of the downgrades this year were on the volatile 2006 vintage SF CDOs (271 tranches totaling US$8.8 billion), although there were also downgrades on 45 tranches totaling US$2.9 billion from 2007 vintage SF CDOs, Moody's said. "The recent rating actions were driven primarily by the extraordinary scope and magnitude of downgrades among the recent vintage RMBS securities backing these SF CDOs," explains Yuri Yoshizawa, Moody's group managing director for US derivatives. Moody's has taken negative ratings action on roughly $77 billion first- and second-lien subprime RMBS assets from the 2006 vintage to date.