MAPT and Alzheimer disease: The ratio of aggregated tau isoforms defines three main groups of tauopathies: 4R (including progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and argyrophilic grain disease), 3R (Pick’s disease), and 3R+4R tauopathies (AD, Down’s syndrome, chronic traumatic encephalopathy) [2,3,4].