HTT and Huntington disease: Abnormal protein aggregates are the common pathological hallmarks for neurodegenerative diseases, including AD (Aβ peptides), Parkinson's disease (PD) (α-synuclein), Huntington's disease (HD) (Huntingtin), and Amyotrophic Lateral Sclerosis (ALS) (superoxide dismutase 1 or TAR-DNA binding protein-43).