4-carboxycholesta-8(9),24-dien-3beta-ol and NAD+ react to form zymosterone (cholesta-8(9),24-dien-3-one), CO2, and NADH + H+. This reaction occurs in the endoplasmic reticulum, catalyzed by NSDHL (Caldas and Herman 2003). Defects in this enzyme are associated with CHILD syndrome (Congenital Hemidysplasia with Ichthyosiform nevus and Limb Defects) (Konig et al. 2000), but cholesterol biosynthesis in cells and tissues from affceted individuals has not been characterized. Instead, the mechanism and stoichiometry of the reaction are inferred from biochemical studies of partially purified rat enzyme (Rahimtula and Gaylor 1972).