GRM5 and fragile X syndrome: Conceptually, the idea of enhanced mGluR-LTD impairing cognition in AD resonates with the well-recognized negative effects of enhanced mGluR-LTD, and mGluR5 functionality, on cognition in a mouse model of Fragile X syndrome (Luscher and Huber, 2010; Ribeiro et al., 2017; Wilkerson et al., 2018).