A loxP site was inserted upstream of exon 13. A lox511 site, a inverted and modified exon 13 with a point mutation (GTC to TTC) that results in the amino acid substitution of phenylalanine for valine at position 617 (V617F) and a loxP site were inserted downstream of of exon 13. Flp-mediated recombination removed the FRT-flanked neomycin resistance cassette inserted downstream of the modified exon 13 and left a lox511 site downstream of the modified exon 13.