FMR1 and fragile X syndrome: Taken together, the findings that 1) mGluR5 stimulates protein synthesis and that 2) FMRP negatively regulates protein synthesis led to the “mGluR theory of fragile X,” which posits that the neurological and psychiatric symptoms of FXS are generated as a consequence of abnormal responses to mGluR activation (Bear et al., 2004).