HTT and Huntington disease: Several studies have shown that proteasome activity is decreased in the affected brain regions of HD patients [67], whereas in yeast, expression of a mutant Htt fragment (Htt72Q or higher) causes aggregate formation, transcriptional dysregulation, cellular toxicity, perturbations in kynurenine pathway metabolites, increased reactive oxygen species (ROS), mitochondrial dysfunction, and defects in endocytosis and apoptotic events [27, 64–66].