MAPT and Alzheimer disease: Cryo-electron microscopy (cryoEM) studies recently showed that paired helical filaments and straight filaments of 3/4R tau from AD contained the same C-shape protofibril structure whereas the 4R tau-structure obtained from CBD was distinctly different.48 Detailed analysis of the tracer binding sites at the ultrastructure level using cryoEM could potentially elucidate the mechanisms behind the different specificity of [18F]PI-2620 and other tau tracers to different isoforms and filaments of tau.