CRISPR-targeting floxed the endogenous exon 2 and inserted an exon 2 with nucleotide substitutions encoding the amino acid substitution lysine to isoleucine at position 72 (K72I). This mutation produces a protein prone to translation error (ep). Flp-mediated recombination removed the FRT-flanked neomycin resistance cassette. Cre-mediated recombination removed the endogenous exon 2.