The HSD17B4 gene encodes an enzyme involved in peroxisomal fatty acid beta-oxidation. It was first identified as a 17-beta-estradiol dehydrogenase . Peroxisomal beta-oxidation of fatty acids is catalyzed by 3 enzymes: acyl-CoA oxidase; the 'D-bifunctional enzyme,' with enoyl-CoA-hydratase and D-3-hydroxyacyl-CoA dehydrogenase activity, and 3-ketoacyl-CoA thiolase. The D- and L-bifunctional proteins have different substrate specificities. The D-bifunctional protein catalyzes the formation of 3-ketoacyl-CoA intermediates from both straight-chain and 2-methyl-branched-chain fatty acids and also acts in shortening cholesterol for bile acid formation. In contrast, the L-specific bifunctional protein does not have the latter 2 activities.