Standard & Poor's Corp. cut its ratings Monday on about $30.9 billion of U.S. prime jumbo residential mortgage-backed securities issued in 2007 because of higher projected losses on high-priced homes.

The rating agency lowered ratings on 620 classes from 49 transactions. Many were lowered from AAA status.

Jumbo loans — those $417,000 and up — generally carry higher interest rates because they are too big to be guaranteed by Fannie Mae or Freddie Mac. S&P said it expects a 40% loss severity on prime jumbo loans originated in 2006 and 2007.

The rating agency said the current credit support to the affected classes was insufficient to maintain their ratings, given the increase in projected losses.

S&P has said it expects loan losses to mirror 1999, which before 2005 was the worst year in the past decade for foreclosures on jumbo loans. The rating agency expects the 2007 losses to top the losses on the 1999 loans, but expects the losses' timing to be more similar to that year than any other.

The collateral on the downgraded deals includes both fixed-rate and adjustable-rate mortgages on one- to four-family residential properties. The downgraded bonds were part of deals put together by Bank of America Corp., Bear Stearns Cos. Inc. (now a part of JPMorgan Chase & Co.), and Continental Home Loans.

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