The targeting construct inserted into intron 12 a lox2272 site, inverted exon 12 containing the nucleotide substitutions that results in the amino acid substitution of aspartic acid with lysine at position 495 (E495K), a loxP site, a lox2272 site, and an FRT-flanked neomycin resistance cassette. Flp-mediated recombination removed the selection cassette and left the modified exon 12 floxed and inverted. (J:309176)