We will specifically explore prionoid mechanisms in the pathologies and pathogenesis of AD (amyloid precursor protein, APP and Tau), HD (Huntingtin, Htt), PD (α-synuclein), and ALS [Fused in Sarcoma (FUS), Superoxide Dismutase 1 (SOD1), and TAR DNA-Binding Protein 43 (TDP-43)]. This evidence concerns the gene MAPT and Alzheimer disease.