7-dehydro Cholesterol (7-DHC) is an immediate precursor of cholesterol. It is reduced to cholesterol by the enzyme 3β-hydroxysterol-Δ7-reductase (DHCR7) in the last step of cholesterol biosynthesis from squalene. Mutations in the gene encoding DHCR7 lead to increased levels of 7-DHC and the neurodevelopmental syndrome Smith-Lemli-Opitz syndrome (SLOS). 7-DHC is elevated in rats treated with the DHCR7 inhibitor AY9944, an animal model of SLOS. 7-DHC is subject to free radical oxidation (autoxidation), giving rise to several oxysterols which contribute to pathogenesis. Serum levels of 7-DHC, largely derived from the liver, are approximately 0.3 μM in healthy individuals. In the skin, photolysis of 7-DHC by ultraviolet light produces vitamin D3 .
