ADH6 and hepatocellular carcinoma: In HCC, five genes were included eventually in our prognostic model: ADH4, ADH6, CYP2C9, CYP4F2, and RDH16. All-trans-retinol dehydrogenase 4 (ADH4) catalyzes the nicotinamide adenine dinucleotide- (NAD-) dependent oxidation of either all-trans-retinol or 9-cis-retinol [39]; retinol dehydrogenase 16 (RDH16) also oxidizes the same substrates with a preference for NAD [40–42] but with higher activity toward cellular retinol-binding protein- (CRBP-) bound retinol than with free retinol [41].