Based on Fmr1 mouse studies of the mGluR theory and the established affinity and specificity of [18F]FPEB to mGluR5, we hypothesized that there would be a significant difference in regional brain concentrations of mGluR5 between the two groups of subjects, validating the use of [18F]FPEB in clinical trials as a tool to confirm target engagement of novel drug agents for mGluR5s in FXS and to monitor dose response. This evidence concerns the gene FMR1 and fragile X syndrome.