MAPT and Alzheimer disease: [11C]PK11195 is a widely used PET tracer that binds primarily to activated microglia in PSP7–9 and other neurodegenerative disorders.10 11 The ligand [18F]AV-1451 is widely used to assess tau pathology in AD and can be informative in PSP (see12 for review) despite lower sensitivity to PSP tau isoforms and off-target binding in some regions.13 14 However, it has not been shown whether either of these PET biomarkers of neuroinflammation and tau pathology predict longitudinal clinical progression in patients with PSP.