Using a modified BAC RP24-359N5 for homologous recombination, a C723 modification in exon 15 was made via site-directed mutagenesis using an FRT-flanked neo and arabinose-induced FLP, which left behind one FRT site, and a loxP-flanked Stop-neo cassette was additionally inserted into intron 1, which resulted in this constitutively expressed null allele that can be cre-excised to yield an expressed C723A mutant allele.