One of the biological manifestations of FXS is elevated levels of metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), (mGluR-LTD) a type of synaptic plasticity which is characterized by a reduction in the synaptic response at the excitatory synapses and overexpression of several proteins including Arc (72, 73). The gene discussed is ARC; the disease is fragile X syndrome.