Standard & Poor's dropped its ratings on $6.84 billion worth of collateralised debt obligation (CDO) debt backed by residential mortgages, and said it may cut an additional $3.59 billion.

The downgrades are on 156 pieces from 36 deals, and 57 tranches of these are still on review for possible further downgrades. An additional 63 tranches were also placed on review for a ratings cut.