PRNP and fatal familial insomnia: Prion disease or transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders (e.g., Creutzfeldt–Jakob (CJD), Gerstmann–Sträussler–Schenker disease (GSS), fatal familial insomnia (FFI), and Kuru), which are characterized by rapidly progressing neuronal loss and extracellular accumulation of the scrapie form of the prion protein (PrPSc), a pathological isoform of the normal cellular prion protein (PrP).