Exon 5 was replaced with one in which nucleotide substitutions result in the amino acid substitutions of alanine for leucine at positions 131 and 132 (L131A, L132A). A floxed hygro cassete inserted upstream of the modified exon 5 was removed by germ line, cre mediated recombination. The substitutions occur within the di-leucine motif. (J:142387)