The construct targeted upstream of exon 6 contains (5' to 3') a loxP site, lacZ gene, FRT site, neomycin resistance cassette, loxP site and FRT site. An additional FRT site was inserted downstream of exon 6. Flp-mediated recombination removed the neomycin resistance cassette and exon 6 but left the lacZ gene intact. Western blot analysis confirmed the absence of protein expression in mouse embryonic fibroblasts. (J:198329)