Apart from prion protein (PrP)2 in the archetypal prion diseases (typified by Creutzfeldt-Jakob disease (CJD)), the evidence that amyloid-β1–42 (Aβ), Tau, and α-synuclein all propagate through the brain is compelling, with clear implications for the pathogenesis of Alzheimer disease, frontotemporal dementias and other tauopathies, and Parkinson disease. The gene discussed is PRNP; the disease is prion disease.