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). The gene discussed is MAPT; the disease is Alzheimer disease.