A loxP was inserted upstream of exon 3 and a STOP cassette followed by a floxed neo cassette and modified genomic sequence for exon 3 through 6. The modified exon 4 contains a nucleotide substitution that results in the amino acid substitution of arginine for lysine at position 225 (K225R). This amino acid is responsible for the ubiquitination-mediated degradation of the peptide. Cre mediated recombination removes the wild-type exon 3 through 6 and the neo cassette allowing for expression of a transcript containing the nucleotide substitution.