A genomic fragment encompassing exons 2 and 3 were replaced with a single loxP site after cre mediated recombination of a single loxP site in intron 3 and the 5' loxP site of a floxed puromycin cassette in intron 1. Neither normal or truncated transcript was detected by Northern blot analysis of homozygous mutant ES cells treated with a DNA damaging agent.