We aimed to determine if (A) FMRP levels are correlated with mGluR5 expression in men with the FM of FXS [2,41,42] and (B) the measurement of FMRP and PET with ([18F]FPEB) is feasible in men with FXS [2,41,42,60,61]. Here, FMR1 is linked to fragile X syndrome.