Exons 4 was replaced with a modified exon 4 in which nucleotide substitutions (T to C and A to G) result in the amino acid substitutions of arginine for a premature stop codon and glutamine at positions 253 and 266 (Stop 253R and Q266R). Exon 5 was replaced with a modified exon 5 in which nucleotide substitutions (A to G and T to C) result in the amino acid substitutions of alanine and proline for threonine and serine at positions 299 and 336 (T299A and S336P). A floxed neo cassette inserted downstream of the modified exon 4 was removed by cre-mediated recombination.