FMR1 and fragile X syndrome: An abnormal morphology and density of dendritic spines was observed in the brain cortex of FXS patients (Irwin et al., 2001) and in the cortex and hippocampus of Fmr1 knockout (KO) mice (Comery et al., 1997; Nimchinsky et al., 2001; Grossman et al., 2010), a model of FXS, suggesting a dysfunction of excitatory synaptic transmission in several brain regions.