An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted upstream of exon 2. An additional loxP site was inserted downstream of exon 4. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 2 through 4 floxed. RT-PCR analysis confirmed the absence of transcript in the retina of homozygous mutant mice.