The biosynthesis of steroid hormones in the gonads and adrenal glands requires the activities of the enzyme 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta HSD) which catalyzes the NAD(+)-dependent dehydrogenation and subsequent delta 5-->delta 4 isomerization of delta 5-3 beta-hydroxysteroids to delta 4-3-ketosteroids. The mouse expresses four isoforms of 3 beta HSD. 3 beta HSD I is expressed in gonads and adrenal glands and appears to be the major steroidogenic form, 3 beta HSDs II and III are expressed in both liver and kidneys, and 3 beta HSD IV has been detected only in kidneys. To determine the genetic relationship between the 3 beta HSD isoforms, we have mapped the chromosomal locations of the four genes by linkage analysis using gene-specific probes derived from the 3' untranslated regions of 3 beta HSD cDNA clones. The four 3 beta HSD structural genes (Hsd3b) are closely linked within a segment of chromosome 3 that is conserved on human chromosome 1. The order of markers on Chr 3 surrounding Hsd3b is: centromere-Gba-(4.4 +/- 2.2)-Hsd3b-(3.3 +/- 1.9)-Tshb-(6.7 +/- 2.7)-Amy-1.