Exons 3 and 4 were replaced with modified exons in which nucleotide substitution result in the amino acid substitutions of phenylalanine for tyrosine at positions 120 and 127 (Y120F, Y127F). Flp-mediated recombination removed the FRT-flanked neomycin resistance cassette with a 5' loxP site inserted between the modified exons. An additional loxP site is located downstream of exon 4. (J:212410)