One of the evident pathological features of neurodegenerative diseases is the accumulation of abnormal proteins, such as α-synuclein in Parkinson disease (PD), amyloid β (Aβ) and tau in Alzheimer disease (AD), huntingtin (HTT) in Huntington disease, polyglutamine-expanded androgen receptor (polyQ-AR) in X-linked spinal and bulbar muscular atrophy (SBMA), and mutant superoxide dismutase 1 (SOD1) in amyotrophic lateral sclerosis (ALS). The gene discussed is HTT; the disease is Alzheimer disease.