Exon 2 was flanked by loxP sites and an FRT-flanked neomycin resistance gene was inserted in intron 1 and an F3-flanked puromycin resistance gene in intron 2. The neomycin and puromycin genes were removed via flp-mediated recombination. Exon 2 was deleted via cre-mediated recombination in the germline. (J:316114)