Exon 14 was replaced with one in which the following nucleotide substitutions have been made: codon 499 (GTG->GGG; V499G), codon 501 (TAT->CAT; Y501H), and codon 500 (ATT->ATC); silent I. A floxed neo cassette used for selection was removed by germ line, cre mediated recombination. (J:145297)