7-dehydro Cholesterol-d7 is intended for use as an internal standard for the quantification of 7-dehydro cholesterol by GC- or LC-MS. 7-dehydro Cholesterol (7-DHC) is an immediate precursor of cholesterol.¹ It is reduced to cholesterol by the enzyme 3β-hydroxysterol-Δ7-reductase (DHCR7) in the last step of cholesterol biosynthesis. It accumulates in Smith-Lemli-Opitz syndrome (SLOS), a disorder characterized by a mutation in the DHCR7 gene and decreased cholesterol levels in bodily tissues and fluids, as well as microcephaly, intellectual disability, and distinctive dysmorphic features.^1,2 7-DHC is highly susceptible to free radical oxidation, giving rise to several oxysterols that may be involved in the pathogenesis of SLOS.¹ 7-DHC levels are increased in brain, liver, and serum in a rat model of SLOS induced by the DHCR7 inhibitor AY 9944 .¹ 7-DHC is also a provitamin that is converted to vitamin D3 by ultraviolet-B (UVB) light in a human skin equivalent system and in isolated human skin samples.^3,4