Melanin is a complex polymer derived from the amino acid tyrosine. Melanin is responsible for determining skin and hair colour and is present in the skin to varying degrees, depending on how much a population has been exposed to the sun historically. Numerous steps are involved in the biosynthesis of melanin. Melanism refers to the deposition of melanin in the tissues of living animals. The chemistry of the process depends on the metabolism of the amino acid tyrosine, the absence of which results in albinism, or lack of pigmentation. Melanism can also occur pathologically, as in a malignant melanoma, a cancerous tumour composed of melanin-pigmented cells.The first step is catalysis of the chemical L-3,4-dihydroxyphenylalanine by tyrosinase. A lack of tyrosine can lead to albinism. Tyrosine is only found in a specialized cells called melanocytes, inside which tiny granules of melanin pigment are contained in vesicles called melanosomes.