Levodopa, an intermediate product of melanin synthesis, is another metabolic pathway that forms catecholamines, which are converted to dopamine, NE and AD by the action of the corresponding transferase enzymes, which can phosphorylate tyrosine and thus regulate tyrosinase activity through the AC/cAMP kinase system and affect melanin content [41]. The gene discussed is TYR; the disease is Alzheimer disease.