HTT and early-onset autosomal dominant Alzheimer disease: Neurodegenerative diseases (NDs) have increasingly been linked to dysfunction of specific proteins, often unique to one disease, e.g. amyloid precursor protein (APP) to Alzheimer's disease (AD), parkin (Park) to Parkinson's disease (PD), huntingtin (HTT) to Huntington's disease (HD), and superoxide dismutase (SOD1) to amyotrophic lateral sclerosis (ALS) (Kaur et al., 2016; Lill, 2016; Nopoulos, 2016; Selkoe and Hardy, 2016).