Its cells contain the pigment melanin. Dark coloration is due to abundant melanin pigment. The key enzyme in melanin synthesis is tyrosinase. Skin whitening agents work by reducing the presence of melanin pigment in the skin. Dark skin is a human skin color that is rich in melanin pigments, especially eumelanin. Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin. Pigment spots arise from expression of the "yellow" gene, whose product produces black melanin. Tyrosine is also the precursor to the pigment "melanin". It is responsible for the distribution of melanin pigment in mammals. In these cases, there has been a genetic mutation in the melanin and pigment delivery. Heterochromia is determined by the production, delivery, and concentration of melanin (a pigment). Two types of melanin, eumelanin and phaeomelanin, are present in birds. With the production of the melanin, the skin color darkens. This is, because green eyes have a strong presence of both melanin and pheomelanin. Eumelanin is the dominant form of melanin found in human skin.