To date, 25 human diseases have been described that are characterized by the deposition of denatured proteins within animal tissues. However, among these, only the prion diseases produce self-propagating infectious material. The common theme of prion diseases, as exemplified by bovine spongiform encephalopathy in cattle and Creutzfeldt–Jakob Disease, Gerstmann-Stra¨ussler-Scheinker Syndrome, and fatal familial insomnia in humans, is their association with a slow buildup of a misfolded protein (PrPSc) in the brain. This deposition is accompanied by a loss of neuronal cells and the characteristic spongiform change. The result of this neurodegenerative process is a debilitating, dementing, and invariably fatal disease.