A loxP was inserted upstream of exon 5 and an FRT flanked neomycin resistance gene cassette with a 3' loxP site was inserted downstream of exon 5. The neo cassette was removed by crossing with FLPe mice (C57BL/6-Tg(CAG-FLPe)36Ito). Exon 5 was deleted through subsequent cre-mediated recombination.