PRNP and fatal familial insomnia: Second, genetic prion diseases, including familial CJD (fCJD), fatal familial insomnia (FFI), and Gerstmann–Sträussler–Scheinker syndrome (GSS), are caused by mutations of the PRNP gene, which increase the tendency for mutated PrPC to be misfolded into the abnormal PrPSc [24, 25].