In spite of a long history of intense investigation, the transmissible spongiform encephalopathies are a poorly understood grouping of neurodegenerative diseases. This family of diseases includes kuru, Creutzfeldt-Jakob disease, and Gerstmann-Sträussler syndrome in humans and scrapie, bovine spongiform encephalopathy ("mad-cow disease") and related syndromes in ruminants and rodents.