PRNP and fatal familial insomnia: PrD, which fall under the genetic aetiologies due to point mutations or octapeptide repeat insertions in the PRNP gene, includes familial CJD (fCJD), Gerstmann–Stäussler–Scheinker (GSS) and fatal familial insomnia (FFI) while sporadic CJD (sCJD) presents with disease aetiology from unknown origins.