Two types of melanin, the red pheomelanin and the black eumelanin, are present in human skin. Valverde et al. (1995) noted that eumelanin is photoprotective, whereas pheomelanin may contribute to UV-induced skin damage because of its potential to generate free radicals in response to ultraviolet radiation. Individuals with red hair have a predominance of pheomelanin in hair and skin and/or a reduced ability to produce eumelanin, which may explain why they fail to tan and are at risk from ultraviolet radiation. In mammals, the relative proportions of pheomelanin and eumelanin are regulated by melanocyte-stimulating hormone (see 176830), which acts via its receptor (MC1R) on melanocytes to increase the synthesis of eumelanin, and also via the product of the agouti locus (AGTI; 600201), which antagonizes this action. [from OMIM]