An FRT-flanked neomycin resistance cassette with a 5' loxP site was inserted upstream of exon 1b. An additional loxP site was inserted downstream of exon 1b. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 1b. Mice fail to express the brain isoform (renin-b), but express the kideny isoform (renin-a). (J:278974)