The endogenous prion protein (PrP) is predominantly an extracellular glycosyl-phosphatidyl-inositol-anchored protein whose function is unknown, but whose conversion to a disease-associated form leads to prion diseases such as Creutzfeldt-Jakob disease in humans, Scrapie in sheep, and Bovine Spongiform Encephalopathy in cattle. The gene discussed is PRNP; the disease is prion disease.