Tests of color vision and unique hue judgments were carried out on a patient with little cone function and abnormal macular pigmentation and optic nerve appearance. These tests revealed weak cone-based dichromatic color vision yet normal red, green, and blue unique hue judgments. In addition, rod input was found to inhibit color discrimination. Genetic analysis revealed the absence oft pigment genes, bur multiple M pigment genes. A mutation that inactivates the encoded pigment was identified in a subset of the M genes. One of these genes with the mutation was in the first position of the array, Thus, these visual deficits and changes in the integrity of the inner retina may be linked to defects in the photopigment genes. (C) 2000 John Wiley & Sons, Inc.