Transmissible spongiform encephalopathy

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are a group of rare, progressive, incurable, and invariably fatal conditions that cause degeneration of the nervous system in humans and other animals, such as cattle and sheep. Prion diseases are caused by abnormally shaped proteins called prions, an idea once considered radical, but now well supported by evidence. Prions consist of a protein called the major prion protein (PrP). Misshapen PrP conveys its abnormal structure to native PrP molecules by a crystallization-like seeding process. Because PrP is continuously produced by a cell and the abnormal proteins stick to each other, they accumulate in the brain, damaging neurons and eventually causing disease.