A loxP site was inserted upstream of exon 2. An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 2. Cre-mediated recombination removed exon 2 and the selection cassette. Western blot analysis confirmed the absence of protein isoforms 1 and 2 expression in mouse embyonic fibroblasts. (J:221071)