In humans the degree of darkness of the skin, hair, and iris of the eye depends primarily on the amount of melanin present. The presence of hemoglobin and carotene in the blood contributes to skin color. Moles and freckles are caused by high local concentrations of melanin; albinism by a lack of melanin; and some birthmarks, e.g., "strawberry marks," by an unusual local proliferation of blood vessels (and hence of hemoglobin) near the skin surface. Tanning of human skin results from an increase of melanin production under the stimulation of ultraviolet light.
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