Second-generation tau PET radioligands, which bind to 4-repeat (4R) tau isoforms in progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD), in addition to mixed 3-repeat and 4R tau in Alzheimer disease (AD), may enable tau imaging for differential diagnosis of parkinsonism. The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.