The reversible, NADP(H)-dependent reduction of all-trans-retinal (atRAL) to all-trans-retinol (atROL) occurs in both rod and cone photoreceptor cells where multiple retinol dehydrogenases (RHDs) are located. RHDs belong to the short-chain dehydrogenase/reductase (SDR) superfamily. RDH8 (located in photoreceptor outer segments) is partially responsible, together with other RDHs, for mediating this reaction. The activity for human RDH8 is inferred from a bovine experiment (Rattner et al. 2000).