DHCR7 and Smith-Lemli-Opitz syndrome: SLOS is caused by mutations in the 3β-hydroxysterol-Δ7 -reductase gene (DHCR7), which encodes the enzyme that converts 7-dehydrocholesterol (7-DHC) to cholesterol in the final step of the cholesterol biosynthesis pathway (Fitzky et al., 1998; Tint et al., 1994; Wassif et al., 1998).