A loxP site was inserted into intron 10 and a lox66 site, duplicates of exon 11 and 12, where aspartic acid codon 370 (GAC) in exon 12 was changed to tyrosine (TAC) (p.D370Y), and a lox71 site were inserted in reverse orientation into intron 12. An FRT site flanked neomycin resistance gene cassette was inserted between endogenous exon 12 and the inserted duplicate exons. The neo cassette was removed through subsequent Flp-mediated recombination. This allele expresses wildtype transcripts/peptide. Only after conditional Cre-mediated inversion of the inserted duplicate exons (with mutated exon 12) and deletion of the endogenous exons 11 and 12 will it express mutated transcript/peptide.