A loxP site and FRT flanked neo cassette were inserted upstream of exon 3 and a loxP site and mutated exon 3 containing a 7 alanine expansion of a 20-residue polyalanine chain were inserted downstream of exon 3 via homologous recombination. Flp mediated recombination removed the neo cassette. (J:155885, J:176573)