A floxed puro/STOP cassette was inserted upstream of the exon containing the initiation codon. Exon 9 was replaced with one in which nucleotide substitution results in the amino acid substitution of lysine for glutamic acid at position 545 (E545K). Flp-mediated recombination removed the floxed neo cassette inserted into intron 9. Cre-mediated recombination is required to achieve expression of the knock-in allele.