A loxP site was inserted upstream of exon 2. Exon 3 was replaced with a modified exon 3 in which nucleotide substitution (A to T) that results in the amino acid substitution of phenylalanine for tyrosine at position 114 (Y114F). Flp-mediated recombination removed the FRT-flanked neo cassette with a 3' loxP site inserted downstream of exon 4. Exons 2 through 4 remain floxed.