DBRS's New York office downgraded 497 classes from 122 RMBS transactions, the majority of which are pre-2006 vintages backed by subprime or alternative A credit collateral.

"Given the combination of current delinquencies and corresponding potential significant losses, along with expectations for future delinquencies and defaults, current credit support is not expected to sufficiently cover the anticipated losses," the rating agency said. "In many cases, subordinate classes have already been impaired, further weakening the available credit support for the remaining senior and mezzanine classes."

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