HTT and early-onset autosomal dominant Alzheimer disease: Due to technical limitations, our initial understanding of NDD was initially restricted to the pathological manifestations of abnormal protein aggregation, such as Aβ protein in Alzheimer’s disease (AD), huntingtin (HTT) protein in Huntington’s disease, α-synuclein in Parkinson’s disease, and neurofilament in amyotrophic lateral sclerosis.