A targeting vector containing a loxP site, cDNA spanning exons 13 through 16, neo cassette, second loxP site and modified exon 13 was inserted into intron 12. The modified exon 13 contains nucleotide substitution(s) that result in the amino acid substitution of asparagine for aspartic acid at position 486 (D486N). Cre-mediated recombination removed the cDNA and neo cassette.