Inhibitors of the last steps of cholesterol biosynthesis such as AY9944 and BM15766 severely impair brain development. Their molecular target is the Delta7-sterol reductase (EC 1.3.1.21), suspected to be defective in the Smith-Lemli-Opitz syndrome, a frequent inborn disorder of sterol metabolism. Molecular cloning of the cDNA revealed that the human enzyme is a membrane-bound protein with a predicted molecular mass of 55 kDa and six to nine putative transmembrane segments. The protein is structurally related to plant and yeast sterol reductases. In adults the ubiquitously transcribed mRNA is most abundant in adrenal gland, liver, testis, and brain. The Delta7-sterol reductase is the ultimate enzyme of cholesterol biosynthesis in vertebrates and is absent from yeast. Microsomes from Saccharomyces cerevisiae strains heterologously expressing the human cDNA remove the C7-8 double bond in 7-dehydrocholesterol. The conversion to cholesterol depends on NADPH and is potently inhibited by AY9944 (IC50 0.013 microM), BM15766 (IC50 1.2 microM), and triparanol (IC50 14 microM). Our work paves the way to clarify whether a defect in the delta7-sterol reductase gene underlies the Smith-Lemli-Opitz syndrome.