HTT and Alzheimer disease: During the onset of neurodegeneration, such as in the case of PD, HD, and AD, normally soluble peptides or proteins (such as Aβ, tau, α‐synuclein, and huntingtin) form a variety of nanoscale sized and heterogeneous soluble oligomeric intermediates and insoluble fibrillar amyloid aggregates, which have a high degree of polymorphism.