HTT and Huntington disease: Amyloid-β (Aβ), the proteolytic product of amyloid-β precursor protein (APP), is associated with AD (Rocchi et al., 2003; Tcw and Goate, 2017); α-synuclein (α-syn) is associated with PD and other synucleinopathies (Stefanis, 2012); and expanded polyglutamine (poly-Q) repeats underpin a range of neurodegenerative disorders, including HD which is caused by mutant forms of huntingtin (mHtt) (Li et al., 1993; Schilling et al., 1995).