A neomycin resistance cassette with an IRES and a 5' loxP site was inserted upstream of the coding sequence in exon 1. An additional loxP site was inserted downstream of exon 1. Cre-mediated recombination removed the coding sequence in exon 1 and the selection cassette. Western blot analysis confirmed the absence of protein expression in testis and mouse embryonic fibroblasts. (J:204996)