A loxP site was inserted upstream of exon 12. An FRT-flanked neo cassette with a 5' loxP site was inserted downstream of exon 12. Flp-mediated recombination removed the neo cassette. Cre-mediated recombination removed exon 12, knocking out the long splice variant. The absence of the long form and expression of the short form was confirmed by western blot analysis. (J:170494)