4-methylcholesta-8(9),24-dien-3-one and NADPH + H+ react to form 4-methylcholesta-8(9),24-dien-3beta-ol and NADP+. This reaction takes place in the endoplasmic reticulum, catalyzed by HSD17B7. Two isoforms of the enzyme due to alternative splicing have been identified but only the first has been tested for enzymatic activity (Marijanovic et al. 2003). The human enzyme has not been studied extensively; molecular details of the reaction are inferred from those worked out in studies of material from rat liver (Gaylor 2002).