PRNP and fatal familial insomnia: Different human prion diseases have different characteristics based on their etiology: sporadic CJD (85–90% of CJD cases); acquired CJDs (less than 1% of CJD cases), including vCJD, iatrogenic CJD, and Kuru; and genetic CJDs (approximately 10–15% of prion disease cases) that occur due to prion gene (PRNP) mutation, including familial CJD, Gerstmann–Sträussler–Scheinker syndrome (GSS), and familial fatal insomnia (FFI).