GRM5 and fragile X syndrome: At the cellular and circuit levels, studies of the FXS mice have focused largely on overactive metabotropic glutamate receptor 5 (mGluR5) signaling (Dölen et al., 2007), increased protein synthesis (Osterweil et al., 2010), and enhanced long-term potentiation (LTP; Auerbach and Bear, 2010).