Standard & Poor's Corp. downgraded $1.55 billion of collateralized debt obligations made up of commercial mortgage-backed securities Tuesday, citing deteriorating credit quality and recent downgrades of the underlying securities.

S&P has lowered ratings on hundreds of billions of dollars of MBS and CDOs the past year as it continues to raise estimates on the amount of losses expected amid still-high delinquencies. On Tuesday, S&P cut the ratings on 22 tranches from four CDOs. Several of those cut were reduced to deep junk territory from investment grade.

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