Hydroxysteroid (17-beta) dehydrogenase 12 (HSD17B12) catalyzes the reaction of 3-oxooctadecanoyl-CoA (3-oxostearoyl-CoA) and NADPH + H+ to form 3-hydroxyoctadecanoyl-CoA and NADP+. This activity of HSD17B12 protein and its localization to the endoplasmic reticulum membrane were established in studies of transfected cells expressing the protein (Moon and Horton 2003). Based on the phenotypes of human subjects deficient in the enzyme, HSD17B3 is thought to catalyze the reduction of androstenedione to testosterone (Geissler et al. 1994). A detailed analysis of sequence similarities among the HSD17B protein family reveals close similarity of specificity-determining features of HSD17B12 and HSD12B3, making HSD17B3 a candidate to catalyze 3-OOD-CoA reduction as well (W. Pearson, unpublished, 2012).