Standard & Poor's Corp. cut its ratings Friday on an additional $4.33 billion of securities backed by U.S. subprime mortgage loans issued in 2005 and 2006.

The ratings agency has been cutting its grades on billions of dollars of residential mortgage-backed deals in recent weeks after boosting its expectation of loan losses as delinquencies keep climbing and home prices keep falling.

The deals downgraded Friday cover about 1% of the U.S. securities backed by first-lien, subprime mortgages rated by S&P in 2005 and 2006. They include 136 classes of securities from 28 transactions.

Several dozen of the downgrades were from an AAA rating.

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