Missense mutations in the cone opsins have been identified as a relatively common cause of red/green color vision defects, with the most frequent mutation being the substitution of arginine for cysteine at position 203 (C203R). When the corresponding cysteine is mutated in rhodopsin, it disrupts proper folding of the pigment, causing severe, early onset retinitis pigmentosa. While the C203R mutation has been associated with loss of cone function in color vision deficiency, it is not known what happens to cones expressing this mutant opsin. Here, we used high-resolution retinal imaging to examine the cone mosaic in two individuals with genes encoding a middle-wavelength sensitive (M) pigment with the C203R mutation. We found a significant reduction in cone density compared to normal and color-deficient controls, accompanying disruption in the cone mosaic in both individuals, and thinning of the outer nuclear layer. The C203R mosaics were different from that produced by another mutation (LIAVA) previously shown to disrupt the cone mosaic. Comparison of these mosaics provides insight into the timing and degree of cone disruption and has implications for the prospects for restoration of vision loss associated with various cone opsin mutations.color vision ͉ cone mosaic ͉ photopigment ͉ retinal imaging ͉ rhodopsin N ormal human color vision is trichromatic and derives from the presence of three spectrally distinct cone types: long-, middle-, and short-wavelength-sensitive (L, M, and S). Redgreen color vision defects are characterized by the absence of either L or M cone function and they affect about one in 12 Caucasian males. Inherited red-green defects can be linked to disruptions at the X-chromosome opsin gene locus, where the Land M-cone opsin genes reside in a head-to-tail array (1). Most of these disruptions involve gross gene rearrangements (2-6). However, it is becoming appreciated that missense mutations underlie a significant proportion of red-green defects (5-8). This raises the question of what impact these missense mutations have on the viability of the cones.Some insight comes from rhodopsin. There are Ͼ130 distinct rhodopsin mutations, involving at least 89 sites within the molecule (data compiled from refs. 9-17.) With rare exception (e.g., refs. 9 and 18), each of these mutations has been associated with either retinitis pigmentosa (RP) or congenital stationary night blindness. Rhodopsin and the cone opsins have structural similarities and similar functional demands. Thus, it is reasonable to hypothesize that mutations in the cone opsins homologous to those in rhodopsin that cause retinitis pigmentosa would affect the viability of the cones.The most common missense mutation in the cone opsins is a substitution of cytosine for thymine at nucleotide position 1101, which corresponds to a substitution of arginine for cysteine at amino acid position 203 (C203R) (Fig. S1). The corresponding mutation in rhodopsin (C187Y) disrupts proper folding of the pigment, causing severe, early onset retinitis pigmentos...