3beta-hydroxysteroid dehydrogenase (3beta-HSD) catalyses a series of obligatory biosynthetic steps in the synthesis of mineralocorticoids, glucocorticoids and sex steroids. Specifically, it converts the 3beta-hydroxysteroids pregnenolone, 17-hydroxypregnenolone, dehydroepiandrosterone, and androstene 3beta,1713-diol into the respective 3beta4 3-ketosteroids: progesterone, 17alpha-hydroxyprogesterone, androstenedione and testosterone. In rodents at least three genes for 3beta-HSD have been found to be expressed. When analysed on southern blots, human DNA shows evidence of six or more sequences with homology to 3beta-HSD probes. So far, two highly homologous but distinct human genes have been cloned, encoding 3beta-HSD type I expressed principally in the placenta, and 3beta-HSD type II expressed in adrenal, ovary and testis. Though the type I and II enzymes differ in their kinetics in detail, they are both capable of converting the same 3beta5 substrates.