The G protein coupled receptor (GPCR) metabotropic glutamate receptor 5 (mGlu5) has been linked to dendritic spine formation, synaptogenesis, cognition, and behavior, as well as to pathological roles in disorders such as fragile X syndrome (1), autism spectrum disorder (2, 3), anxiety, and depression (4, 5). This evidence concerns the gene GRM5 and fragile X syndrome.