The results of this study confirm the reduced availability of mGlu5 receptors in FXS; using the cerebellum as a reference region, we found significantly lower [18F]FPEB BP in the FMR1-KO mice compared to the control mice in key brain areas, consistent with mGluR5 distribution and implicated in the symptomatology of FXS. This evidence concerns the gene GRM5 and fragile X syndrome.