Tyrosinase is a key enzyme involved in the initial stages of melanin biosynthesis. Tyrosinase catalyzes the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA). Oxidation reactions of DOPA to L-Dopaquinone and 5,6-dihydroxyindole (DHI) to indole-quinone occur spontaneously at physiological pH. Tyrosinase is expressed in melanin-producing cells such as melanocytes, which are primarily localized in the skin, hair bulbs and eyes. Low levels of tyrosinase mRNA was also detected in the human substantia nigra, but immunohistochemically unreactive. Since melanomas arise from melanocytes, there is evidence that tyrosinase is expressed in malignant melanomas. Studies have shown that tyrosinase is a sensitive and reliable marker to assess melanocytic lesions in paraffin-embedded tissue.