An engineered A to T mutation in exon 1 (c.65A>T) results in a lysine to methionine substitution at amino acid 22 in the encoded peptide (p.K22M). A floxed puromyin resistance gene cassette was inserted upstream of exon 1. The puro cassette was excised by crossing mutant animals to the cre-expressing line Tg(CMV-cre)1Nagy.