HTT and juvenile Huntington disease: The majority of neurodegenerative diseases are characterized by the accumulation of misfolded proteins in the central nerve system, e.g., amyloid-β (Aβ) and tau aggregates in Alzheimer disease (AD), SOD1 pathology in amyotrophic lateral sclerosis (ALS), and mutated huntingtin (HTT) in Huntington disease (HD), and accompanied by a progressive loss of neurons, e.g., the loss of dopaminergic (DA) neurons in PD (Parkinson’s disease) in the affected regions [142].