MAPT and early-onset autosomal dominant Alzheimer disease: Alzheimer's disease (AD) is biologically defined by the presence of neuritic plaques of amyloid‐beta peptide (A42) in the cerebral cortex and by neurofibrillary tangles of hyperphosphorylated tau (P‐tau) protein, which may be measured in vivo through Positron Emission Tomography Scan (PET‐Scan).