PRNP and Gerstmann-Straussler-Scheinker syndrome: Similarly, in genetic prion diseases such as familial CJD (fCJD), fatal familial insomnia (FFI), and Gerstmann-Sträussler-Scheinker disease (GSS), mutations within the PRNP gene encoding PrP are believed to promote the spontaneous formation of PrPSc.