Prion diseases (PrD) or transmissible spongiform encephalopathies (TSE) are invariably fatal and pathogenic neurodegenerative disorders caused by the self-propagated misfolding of cellular prion protein (PrPC) to the neurotoxic pathogenic form (PrPTSE) via a yet undefined but profoundly complex mechanism. The gene discussed is PRNP; the disease is prion disease.