The enzyme 3 beta-hydroxysteroid dehydrogenase (3 betaHSD) is essential for all steroid hormone biosynthesis. Six distinct 3 betaHSD cDNAs in the mouse (3 betaHSD I-VI) have been isolated previously. This study reports the isolation of genes or partial genes encoding the 3 betaHSDI, II, III, and IV isoforms. Characterization of the genes revealed that they consist of four exons, the same structure that has been observed for characterized human 3 betaHSD genes. Primer extension and nuclease S1 analysis identified the start sites of transcription of Hsd3b -1 and -4. The proximal promoter regions of Hsd3b-1 and -4 were sequenced and putative cis-acting sequences were determined. Previously, we reported that the then known 3 betaHSD genes (3 betaHSD I-IV) were located in a small region of mouse chromosome 3. To analyze this locus further, six yeast artificial chromosome clones containing the 3 betaHSD sequence were identified. One clone appears to contain the complete 3 betaHSD locus within its 1,400-kbp insert. Further analysis of this YAC, along with analysis of mouse genomic DNA by pulsed-field gel electrophoresis, suggests all members of the 3 betaHSD gene family may be contained on a 400-kbp fragment.