Elevated 7-dehydrocholesterol levels occur as the result of a deficiency of the enzyme delta-7-sterol reductase, the penultimate enzyme of mammalian sterol biosynthesis that converts 7-dehydrocholesterol (7-DHC) to cholesterol. This enzyme removes the C(7-8) double bond introduced by the sterol delta8-delta7 isomerases. In addition, its role in drug-induced malformations is known: inhibitors of the last step of cholesterol biosynthesis such as AY9944 and BM15766 severely impair brain development. Homozygous mutations in the gene encoding for this enzyme are the molecular genetic cause of Smith-Lemli-Opitz Syndrome.