A 9 base pair in-frame deletion was introduced into exon 54 between nucleotides 7666-7674, deleting amino acids serine, arginine and isoleucine. This corresponds to the 7636del9 human ATM mutation, one of the most common deletion mutations found in human patients. A PGK neo/gpt cassette was also inserted into the intron adjacent to exon 54 and subsequently deleted via a Cre/loxP recombination event. As determined by immunoblotting, no ATM-kinase activity was detected. (J:69726)