Prior studies have linked dysfunction of metabotropic glutamate receptors (mGluR) to idiopathic autism spectrum disorder (IASD) and fragile X syndrome (FXS). In this study, the investigators evaluated mGluR₅ expression in participants with IASD, FXS, and typical development (TD) using positron emission tomography (PET). The Institute for Neurodegenerative Disorders (IND) in New Haven, Connecticut enrolled 7 participants with FXS, 1 participant with FXS with allele size mosaicism, and 12 TD participants. 4 participants with FSX, 6 participants with ASD, and 7 TD participants were recruited from Johns Hopkins University (JHU) in Baltimore, Maryland. Investigators at both sites injected participants with a 185 MBq (5 mCi) intravenous bolus of a mGluR₅ PET ligand, 3-[¹⁸F]fluoro-5-(2-pyridinylethynyl)benzonitrile, prior to PET to quantitatively measure the density and distribution of mGluR5s in the brain regions of participants of both sexes with IASD and TD and men with FXS.

Data for participants from IND were expressed as the standard uptake value ratio (SUVR) with the cerebellum as the reference region. Data for participants from JHU were represented as regional nondisplaceable binding potentials (BPNDs) by reference tissue graphical analysis (RTGA) with the cerebellar white matter as the reference region.