MAPT and early-onset autosomal dominant Alzheimer disease: Although the underlying cause of Alzheimer’s disease (AD) remains poorly understood, it is believed to occur when abnormal amounts of the proteins amyloid beta and tau aggregate in the brain, forming extracellular plaques (amyloidopathy) and neurofibrillary tangles (tauopathy) that result in a progressive loss of neuronal function and dementia (Hardy 2009; Spillantini and Goedert 2013).