An FRT-flanked neomycin resistance cassette with a 5' loxP site was inserted upstream of a modified exon 12, in which nucleotide substitutions (TGT to GCC) results in the amino acid substitution of alanine for cysteine at position 472 (C472A). This mutation inactivates the protease activity. Flp-mediated recombination removed the selection cassette and left the modified exon 12 is floxed. (J:235331)