PRNP and fatal familial insomnia: Prion diseases, such as Creutzfeldt-Jakob disease (CJD), fatal familial insomnia (FFI), or Gerstmann-Sträussler-Scheinker syndrome (GSS), exhibit a template-induced conversion of the cellular prion protein (PrPC) into an abnormally folded isoform, termed PrP scrapie (PrPSc).