Insertion of a floxed PGK-neomycin cassette deleted the exon encoding the BH3 domain. Cre-mediated recombination in vivo subsequently removed the inserted cassette. In the final allele a single loxP site remains in place of the exon. Immunoblot analysis using a rat monoclonal antibody did not detect full-length protein, but did detect a small amount of a truncated polypeptide in spleen cells from homozygous mutant mice.