HTT and Huntington disease: In Parkinson disease and HD, damaged mitochondria and causative proteins (alpha-synuclein and to a much smaller extent, tau in PD, and mutant huntingtin in HD) accumulate in affected neurons, indicating a problem with autophagy or the ubiquitin/proteasome system (Bloom, 2014; Atik et al., 2016; Zhao et al., 2016; Chiasseu et al., 2017; Zhang et al., 2018; Finkbeiner, 2019; Harrison et al., 2019).