FMR1 and fragile X syndrome: Research examining metabolic systems in FXS has burgeoned following the finding that silencing FMRP in the Fmr1-KO mouse results in amplified signaling through specific G protein coupled receptors (GPCRs) – group I metabotropic glutamate receptors (mGluR1 and mGluR5) [17] and muscarinic acetylcholine receptors (mAChRs) [18].