A likely downstream target, the small GTPase Rac1, acts as a molecular switch in structural and functional synaptic plasticity, and is of interest in the context of FXS hyperexcitability (Lee et al., 2003; Schenck et al., 2003; Bongmba et al., 2011; Goto et al., 2013, 2014; Tejada-Simon, 2015). The gene discussed is RAC1; the disease is fragile X syndrome.