A loxP site was inserted upstream of exon 1. An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted downstream of exon 1. Cre-mediated recombination removed the selection cassette and exon 1. Western blotting and immunohistochemistry confirmed the absence of protein (even of a smaller size) in the cornea of homozygous mutant mice.