Normal color vision in humans is trichromatic, being based on 3 classes of cone that are maximally sensitive to light at approximately 420 nm (blue cones; 613522), 530 nm (green cones; 300821), and 560 nm (red cones; 300822). Comparison by neural circuits of light absorption by the 3 classes of cone photoreceptors allows perception of red, yellow, green, and blue colors individually or in various combinations. Dichromatic color vision is severely defective color vision based on the use of only 2 types of photoreceptors, blue plus green (protanopia) or blue plus red (deuteranopia; see 303800). Anomalous trichromacy is trichromatic color vision based on a blue, green, and an anomalous red-like photoreceptor (protanomaly), or a blue, red, and an anomalous green-like photoreceptor (deuteranomaly). The color vision defect is generally mild but may in certain cases be severe. Common variation in red-green color vision exists among both normal and color-deficient individuals (review by Deeb, 2005).