HTT and Huntington disease: Ubiquitin may be involved in HD pathogenesis through the relationships between HTT, ATN1, and ATXN2. The ubiquitin-proteasome system (UPS) has been linked extensively to the pathogenesis of neurodegenerative diseases, including HD and other SCAs (Williams and Paulson, 2008; Arrasate and Finkbeiner, 2012; Atkin and Paulson, 2014; Dantuma and Bott, 2014; Ortega and Lucas, 2014; Bettencourt et al., 2016).