Hereditary color "blindness" (which can run the gamut from a mild color deficiency to severe color perceptual loss) is most commonly due to defects in the photochemicals in the cone photoreceptors. The milder forms involve shifts in the wavelength that the pigment absorbs the most. The more severe forms involve the functional loss of one photopigment. These disorders are genetic in nature. However, there are also acquired cases of color blindness caused by neuronal damage that is post-receptor, such as in optic nerve disease. Less common is color blindness due to cortical damage, such as achromatopsia.