GRM5 and fragile X syndrome: 3-[18F]fluoro-5-(2-pyridinylethynyl)benzonitrile ([18F]FPEB), a novel, specific mGluR5 ligand to quantitatively measure the density and distribution of mGluR5s in the brain regions of humans with FXS through PET [21,22,23,24,25,26,27,28,29,30] (Figure 1), constitutes an effective tool to confirm target engagement of mGluR5s of relevance to clinical trials of NAMs for individuals with FXS [31].