GRM5 and fragile X syndrome: In a recent study, Brašić and colleagues17 used the same [18F]FPEB tracer in males with FXS, slightly younger than those in the present study, though incorporating different acquisition protocols and data analysis methods for PET across collaborating sites; they found mGluR5 density was significantly reduced in multiple brain regions including the cingulate, cortex, thalamus and striatum, compared to age-matched males with typical development, thereby supporting the use of [18F]FPEB to measure drug occupancy in clinical trials for mGLuR5 in FXS.