MAPT and early-onset autosomal dominant Alzheimer disease: Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by two major pathological hallmarks: (1) the formation of extracellular β-amyloid (Aβ) plaques; (2) the accumulation of intracellular neurofibrillary tangles (NFTs) composed of phosphorylated tau.