Three beta-hydroxysteroid dehydrogenase/delta 5-delta 4-isomerase (3 beta-HSD) catalyze the oxidative conversion of delta 5-3 beta-hydroxysteroids to the delta 4-3-keto configuration and is therefore essential for the biosynthesis of all classes of hormonal steroids, namely progesterone, glucocorticoids, mineralocorticoids, androgens, and estrogens. Using human 3 beta-HSD cDNA as probe, a human 3 beta-HSD gene was isolated from a lambda-EMBL3 library of leucocyte genomic DNA. A fragment of 3 beta-HSD genomic DNA was also obtained by amplification of genomic DNA using the polymerase chain reaction. The 3 beta-HSD gene contains a 5'-untranslated exon of 53 base pairs (bp) and three successive translated exons of 232, 165, and 1218 bp, respectively, separated by introns of 129, 3883, and 2162 bp. The transcription start site is situated 267 nucleotides upstream from the ATG initiating codon. DNA sequence analysis of the 5'-flanking region reveals the existence of a putative TATA box (ATAAA) situated 28 nucleotides upstream from the transcription start site while a putative CAAT binding sequence is located 57 nucleotides upstream from the TATA box. Expression of a cDNA insert containing the coding region of 3 beta-HSD in nonsteroidogenic cells shows that the gene encodes a single 42-kDa protein containing both 3 beta-hydroxysteroid dehydrogenase and delta 5-delta 4-isomerase activities. Moreover, all natural steroid substrates tested are transformed with comparable efficiency by the enzyme. In addition to its importance for studies of the regulation of expression of 3 beta-HSD in gonadal as well as peripheral tissues, knowledge of the structure of the human 3 beta-HSD gene should permit investigation of the molecular defects responsible for 3 beta-HSD deficiency, the second most common cause of adrenal hyperplasia in children.