Two classes of CWALT Inc. Alternative Loan Trust series 2006-18CB mortgage pass-through certificates have been placed on Rating Watch Negative by Fitch Ratings.The affected securities are classes B-3 and B-4. Fitch also affirmed the ratings on four classes from the transaction. The negative rating actions were based on "early trends in the relationship" between serious delinquency and credit enhancement, the rating agency said. The certificates are collateralized chiefly by conventional 15- and 30-year fixed-rate mortgage loans originated under Countrywide Home Loans Inc.'s standard underwriting guidelines and extended to prime borrowers, or under its expanded underwriting guidelines and extended to alternative-A borrowers, Fitch reported.

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