Distinct patterns of clinical trajectory, cognitive profile, serum neurofilament light chain level, genetic, and volumetric magnetic resonance imaging measures helped to distinguish the clinical subtypes of progressive supranuclear palsy and corticobasal syndrome; clinical trajectory and serum neurofilament light chain levels distinguished Parkinson disease from progressive supranuclear palsy and corticobasal syndrome. The gene discussed is NEFL; the disease is Parkinson disease.