FMR1 and fragile X syndrome: Electrophysiology studies on Fmr1 KO mice hippocampi first identified a peculiar alteration of synaptic plasticity: in particular, long-term depression induced by metabotropic glutamate receptors (mGluR-LTD) was abnormally enhanced [40], leading to the “mGluR theory” of Fragile X Syndrome [41].