A targeting construct with cre recombinase and an Frt-flanked neo cassette was inserted by homologous recombination into the Cyp11b2 (aldosterone synthase, AS) locus, replacing most of the coding region, retaining only exon 1, intron 1 and the first part of exon 2. Germline animals were crossed to Flpe deleter mice to remove the neo cassette. Mice homozygous for the allele are effectrively null for Cyp11b2 function.