Isomerization of zymosterol to cholesta-7,24-dien-3beta-ol is catalyzed by EBP in the endoplasmic reticulum. The biochemical details of the reaction have been established through studies of purified rat EBP; the role of the human enzyme has been established through studies of patients deficient in it (Derry et al. 1999; Braverman et al. 1999).