11-cis-specific retinol dehydrogenase (11cRDH) activity found in chicken retina mediates the oxidation of 11-cis-retinol (11cROL) to 11-cis-retinal (11cRAL) using NADP+ as cofactor (Mata et al. 2002). The chicken protein that possesses this activity has not yet been identified. The actual enzyme responsible remains to be identified in human, even as evidence exists for this activity and for the alternative retinoid (visual) cycle in cones (Wang & Kefalov 2009). 11cRAL is the visual chromophore that is able to bind to opsins.