Site-directed mutagenesis within exon 2 generated missense mutations at codons 18 and 23, resulting in the substitutions of both serines with alanines (p.S18A, p.S23A). A single loxP site remained in intron 4 after a loxP site flanked neomycin resistance gene cassette was excised via Cre-mediated recombination. Sequencing verified that only the S18/23A mutations were present in mutant mice.