Multiple lines of evidence suggest that dysfunction of the metabotropic glutamate receptor subtype 5 (mGluR₅) plays a role in the pathogenesis of autism spectrum disorder (ASD). Yet animal and human investigations of mGluR₅ expression provide conflicting findings about the nature of dysregulation of cerebral mGluR₅ pathways in subtypes of ASD. The demonstration of reduced mGluR₅ expression throughout the living brains of men with fragile X syndrome (FXS), the most common known single-gene cause of ASD, provides a clue to examine mGluR₅ expression in ASD. We aimed to (A) compare and contrast mGluR₅ expression in idiopathic autism spectrum disorder (IASD), FXS, and typical development (TD) and (B) show the value of positron emission tomography (PET) for the application of precision medicine for the diagnosis and treatment of individuals with IASD, FXS, and related conditions. Two teams of investigators independently administered 3-[¹⁸F]fluoro-5-(2-pyridinylethynyl)benzonitrile ([¹⁸F]FPEB), a novel, specific mGluR₅ PET ligand to quantitatively measure the density and the distribution of mGluR₅s in the brain regions, to participants of both sexes with IASD and TD and men with FXS. In contrast to participants with TD, mGluR₅ expression was significantly increased in the cortical regions of participants with IASD and significantly reduced in all regions of men with FXS. These results suggest the feasibility of this protocol as a valuable tool to measure mGluR₅ expression in clinical trials of individuals with IASD and FXS and related conditions.