PRNP and human prion disease: The finding that prion protein is present in normal and TSE-infected brain at similar levels [3], [4] suggests that the “cellular” prion protein (PrPC) constitutes a precursor of the “scrapie” prion protein (PrPSc) causing TSE such as bovine spongiform encephalopathy (BSE) in cattle or Creutzfeldt-Jakob disease (CJD) in humans.