MC1R, a Melanocortin Receptor, mediates the effects of melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH). MSH and its receptor, MC1R, are key regulators of melanogenesis. They control the relative proportions of red pheomelanin and black photoprotective eumelanin in the skin by increasing the production of eumelanin. Non-functional, truncated forms of the receptor lead to lighter coat color in animals. In contrast, constitutively active receptors lead to darker color. Pheomelanin generates free radicals in response to UV radiation, and elevated levels of pheomelanin have been associated with higher risk of UV-induced skin damage. Therefore, MC1R is a major determinant of sun sensitivity and genetic risk for melanoma and other skin cancers. Expression of melanocortin 1 receptor has been reported primarily in adrenal, skin, and testis. ESTs have been isolated from breast, placenta, and testis libraries.