At a cerebral level, in FXS conditions (Irwin et al., 2000) as in the animal model (Irwin et al., 2000; Nimchinsky et al., 2001; Bilousova et al., 2009), FMRP deficiency leads to the neuronal immaturity phenotype caused by proteomic defects. The gene discussed is FMR1; the disease is fragile X syndrome.