A serine to alanine substitution at amino acid 1152 (S1152A) was introduced into exon 11 and a floxed neo cassette was inserted upstream of exon 11 via homologous recombination. Mating with mice expressing cre recombinase in the germ line excised the floxed neo. This allele results in a mutation of the ataxia telangiectasia mutated homolog (ATM) phosphorylation site. Western blot analysis indicated that mutant protein was expressed at levels similar to those of wild type.