PRNP and Creutzfeldt Jacob disease: The second most common variant (10–15% of cases) is genetic prion disease, which is related to pathogenic mutations in PRNP and encompasses three distinct clinical–pathological phenotypes, the genetic form of CJD (gCJD), fatal familial insomnia (FFI), and Gerstmann–Sträussler–Scheinker syndrome (GSS).