HTT and Parkinson disease: The normally soluble proteins of unrelated sequences, such as Aβ in Alzheimer’s disease, α-synuclein (α-syn) in Parkinson’s disease (PD), and polyglutamine (polyQ) expanded huntingtin in Huntington’s disease (HD), form morphologically similar amyloids that possess a core structure of characteristic cross β-conformation [1, 2], [3], [4].