The correction of mGluR-LTD and behavioral symptoms in fmr1 KO mouse models suggests that the a biomarker to measure mGluR5 expression in the living human brain represents a means to apply precision molecular medicine to ameliorate behavioral symptoms of FXS and possibly other subtypes of ASD [9,10,17,18,19,20,21,22,23,24]. This evidence concerns the gene GRM5 and fragile X syndrome.