Standard & Poor's Corp. cut its ratings Tuesday on $32.1 billion of securities backed by alternative-A mortgages issued from 2005 through 2007, saying the bonds probably would not be able to cover projected losses due to higher delinquencies.

The rating agency downgraded 450 classes from 44 residential mortgage-backed deals and removed 305 of those ratings from watch for downgrading. S&P affirmed its ratings on another 177 classes from 39 of those deals.

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