An FRT-flanked beta-geo cassette with a transmembrane domain and a 3' loxP site. An additional loxP site was inserted downstream of exon 5. Flp-mediated recombination removed the beta-geo cassette. Cre-mediated recombination removed exons 3 through 5 floxed. Western blot analysis confirmed the absence of protein expression in E16.5 brains. (J:199510)