TSPO and Alzheimer disease: [11C]PK11195 binds to the 18 kDa translocator protein which is overexpressed in activated microglia, but also in other cell types, like astrocytes and vascular smooth muscle cells, although it has been found selective for activated microglia over reactive astrocytes.42 There are also caveats for [18F]AV-1451, namely, its off-target binding (monoamine oxidase, choroid plexus and neuromelanin) and lower affinity for PSP tau compared with AD-related tau (see Malpetti et al and Leuzy et al7 12 for discussion regarding limitations of this ligand).