A loxP site was inserted upstream of exon 1. A loxP- and FRT-flanked neomycin resistance cassette was inserted downstream of exon 1. Flp-mediated recombination removed the neomycin resistance cassette and left exon 1 floxed. Cre-mediated recombination removed exon 1. Western blot analysis confirmed the absence of protein expression in mouse embryonic fibroblasts. (J:199753)