Normal human colour vision is trichromatic, based on 3 types of cones that are maximally sensitive to light at approximately 420 nm (blue cones), 530 nm (green cones), and 560 nm (red cones). Neural circuits compare light absorbed by these 3 cone types to perceive those primary colours and combinations of them. Colour vision deficiencies result from genetic mutations that affect the expression of the full complement of cone photoreceptors and are classified by severity of deficiency.

Anomolous trichromacy is the mildest form. Although affected individuals express all three cones, the way the cones process the primary colours is aberrant so discriminating various colours is difficult. Anomolous trichromacy is subdivided into protanomaly (affects red cones), deuteranomaly (affects green cones) and tritanomaly (affects blue cones). Dichromacy is the next severest colour vision deficiency. Dichromats have reduced colour vision based on the use of only 2 types of cone photoreceptors. Dichromacy is subdivided into protanopia (no functional red cones), deuteranopia (no functional green cones) and tritanopia (no functional blue cones). Monochromacy is the severest form of colour vision deficiency in which colour discrimination is absent due to dysfunctional or non-functional cones. All vision is therefore mediated by rods which otherwise usually function only in night conditions (see reviews Deeb 2005, Simunovic 2010).

Defects in OPN1MW cause X-linked cone dystrophy type 5 (COD5; MIM:303700), a retinal dystrophy characterized by progressive degeneration of cone photoreceptors but with preserved rod function. The W177R missense mutation in both the LW-sensitive (red) and MW-sensitive (green) cone opsin genes results in protein misfolding and retention in the endoplasmic reticulum which can lead to COD5 (Gardner et al. 2010).