The targeting vector introduced into intron 1 a loxP site, an FRT site, splice acceptor, IRES, hygromycin resistance cassette, polyadenylation sequence, zeomycin resistance cassette, and FRT site. An additional loxP site was isnerted downstream of exon 2. Cre-mediated recombination removed the selection cassettes and exon 2. (J:258043)