GRM5 and fragile X syndrome: We showed that 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 through PET (Figure 1) [25] may be a promising means to obtain quantitative measurements of mGluR5 expression in individuals with IASD or FXS for clinical trials and other investigations [14,17,23,26].