GRM5 and fragile X syndrome: In addition, studies using fmr1−/− mice have characterized the abnormally increased group I mGluR (especially mGluR5) activation-induced protein synthesis [72,86], which underlies several typical abnormalities observed in FXS, such as enhanced mGluR-dependent long-term depression (LTD) [87], induction of elongated/immature dendrites [88], and increased susceptibility to audiogenic seizure [89], and so on.