A loxP site, a mutated exon 2 containing a G to C change at position 423 resulting in a lysine to asparagine mutation at amino acid 141 (K141N), an FRT-flanked neomycin selection cassette, and a loxP site replaced exon 2. Cre-mediated recombination removed the mutated exon 2 and the neomycin gene resulting in a null allele. Western blot analysis confirmed absence of protein.