The targeting vector replaced exon 1 with a loxP site, splice acceptor, cDNA for exons 1 through 4, polyadenylaiton sequence, FRT-flanked neomycin resistance cassette, loxP site, splice acceptor, and modified exon 1 with the amino acid substitution of glycine with serine at position 12 (G12S). Cre-mediated recombination removed the cDNA allowing for expression of the mutant form. (J:259739)