HTT and Alzheimer disease: Insoluble aggregates can be formed as a result of covalent cross-links among peptide chains, as in the case of amyloid-β-peptide (Aβ) in Alzheimer disease (AD), α-synuclein in Parkinson disease (PD), huntingtin in Huntington disease (HD), and SOD1 in amyotrophic lateral sclerosis (ALS).