Prion diseases are fatal disorders affecting the nervous system of several species, characterized by: (1) progressive loss of neurons; (2) lack of classical inflammation; (3) appearance of vacuolation in the neuropil (“spongiform encephalopathy”); (4) deposition of abnormal conformers of prion protein (PrP); and (5) transmissibility in most forms of the disease (hence the terminology “transmissible spongiform encephalopathy”, TSE). The gene discussed is PRNP; the disease is prion disease.