MAPT and Alzheimer disease: These diseases are caused by abnormally folded proteins (pathogenic prions PrP<sup>Sc</sup> in prion diseases, β-amyloids/Aβ and Tau in Alzheimer's disease, α-synuclein in Parkinson's disease) that display β-sheet-enriched structures, propagate and accumulate in the nervous central system, and trigger neuronal death.