The prion diseases, sometimes referred to as the "transmissible spongiform encephalopathies," include kuru, Creutzfeldt-Jakob disease, and Gerstmann-Sträussler-Scheinker disease of humans as well as scrapie and bovine spongiform encephalopathy of animals. For many years, the prion diseases were thought to be caused by viruses despite intriguing evidence to the contrary. The unique characteristic common to all of these disorders, whether sporadic, dominantly inherited, or acquired by infection, is that they involve the aberrant metabolism of the prion protein (PrP). In many cases, the cellular prion protein is converted into the scrapie isoform by a posttranslational process that involves a conformational change. Often, the human prion diseases are transmissible to experimental animals and all of the inherited prion diseases segregate with PrP gene mutations.