A C to G single-base pair substitution that changes the Tyrosine 632 codon (TAC) to a stop codon (TAG; Y632X) was introduced by site-directed mutagenesis in exon 3. A neomycin selection cassette flanked by FRT sites was inserted 199 bp upstream of the native stop codon into a non-conserved region of 3-UTR in exon 2 which is devoid of potential microRNA-binding sites. The selection cassette was excised via flp-mediated recombination. mRNA is stable and mRNA levels are similar to wild-type levels.