The pollutant methylmercury is a globally distributed neurotoxin and an endocrine system disruptor. Larger fish tend to bioaccumulate higher methylmercury levels. The most toxic forms of mercury are its organic compounds, such as dimethylmercury and methylmercury. Mercury methylation is the process of forming methylmercury (MeHg). Ingested methylmercury is readily and completely absorbed by the gastrointestinal tract. Mercury can then enter into the human food chain in the form of methylmercury. Since fish are less efficient at depurating than accumulating methylmercury, fish-tissue concentrations increase over time. Niigata Minamata was caused by methylmercury poisoning in the Agano River basin. In some cases, organometallic forms, such as methylmercury and tetraethyl lead, can be extremely toxic. Fish and other aquatic species are the main source of human methylmercury exposure. For example, the highly toxic methylmercury is produced by the action of methylcobalamin. Like other fish, albacore accumulate methylmercury in body tissue over time. Dimethylmercury is metabolized after several days to methylmercury. High-level exposure to methylmercury is known as Minamata disease. However, currently there are few direct anthropogenic sources of methylmercury pollution in the United States.