All subtypes of prion disease, including Creutzfeldt-Jakob disease (CJD), fatal familial insomnia (FFI), Kuru, and Gerstmann-Sträussler-Scheinker (GSS) disease in humans as well as scrapie, chronic wasting disease, and bovine spongiform encephalopathy (or mad cow disease) in animals, are caused by PrP (1). This evidence concerns the gene PRNP and prion disease.