This excerpt taken from the PNC 10-Q filed May 11, 2009.
OTTICommercial Mortgage-Backed Securities
During the first quarter of 2009, we recognized credit losses of $5 million on three commercial mortgage-backed securities. To measure credit losses we compile relevant collateral details and performance statistics on a security-by-security basis. The securities are then processed through a series of pre-established filters that include minimum thresholds for external credit ratings, that ratio of delinquencies to current credit enhancement, and market price.