The human transmissible spongiform encephalopathies (TSEs), or prion diseases, are a group of rapidly progressive disorders characterized by a spectrum of clinical abnormalities that include cognitive impairment, ataxia, myoclonus, and visual, pyramidal, and extrapyramidal signs. They share a spongiform (vacuolar) degeneration and variable amyloid plaque formation. Examples of TSEs are kuru, an infectious disease; Creutzfeldt-Jakob disease (CJD), which may take an infectious, genetic, or sporadic form; and Gerstmann-Sträussler-Scheinker disease (GSS) and fatal familial insomnia (FFI), rare familial disorders. With the exception of FFI, all of these disorders have been experimentally transmitted to nonhuman primates and laboratory rodents. The pathogenic PrP protein accumulating in the brain of TSE patients is a protease-resistant and insoluble product of a precursor protein molecule of unknown function that is encoded by the PRNP gene on chromosome 20. Different mutations in this gene are responsible for various phenotypes of TSE in its familial form, and a polymorphism at codon 129 controls susceptibility to the infectious and perhaps sporadic forms of disease. TSEs are transmissible amyloidoses in which the host-encoded protein has the propensity to acquire a beta-sheet conformation and produce amyloid; the accumulation of amyloid eventually destroys the neurons and induces the deadly disease.