We aimed to (A) compare and contrast mGluR5 expression for participants of both sexes with IASD [14,23] and typical development (TD) [14,25] and men with FXS [17,26] and (B) show the value of PET with ([18F]FPEB) for the application of precision medicine for the diagnosis and treatment of individuals with IASD, FXS, and related conditions [9,17,24,28]. This evidence concerns the gene GRM5 and fragile X syndrome.