The affected signaling cascades include the mammalian target of rapamycin (mTOR) [29,30,31,32,33], the microtubule-associated protein kinase (MAPK) [34], the protein kinase A (PKA) [35], and the extracellular signal-regulated kinase (ERK) pathways, which may contribute to metabotropic glutamate receptor dependent long-term depression (mGluR-LTD) and to the neurobehavioral symptoms of FXS in Fmr1 knockout (KO) mouse models [7,15,16,36,37,38]. Here, FMR1 is linked to fragile X syndrome.