A loxP site was inserted upstream of exon 2. A neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 2. Cre-mediated recombination remove exon 2 and the neomycin resistance cassette. Western blot analysis confirmed the absence of exon 2-containing isoform(s) but over-expression of isoform ZnF3/4 in the E15 forelimb. (J:199127)