PRNP and Gerstmann-Straussler-Scheinker syndrome: These may arise spontaneously within the CNS (such as sporadic Creutzfeldt-Jakob disease (CJD)) or are associated with polymorphisms within the PRNP gene (which encodes PrPC) which some consider predisposes the prion protein to abnormally fold into the disease-specific isoform (such as Gerstmann-Straussler-Scheinker syndrome).