Cortical neurons from Fmr1 knock out (Fmr1 KO) animal models of FXS (Comery et al., 1997) and FXS patients (Irwin et al., 2000) display an increased density of dendritic spines, with a long and thin morphology reminiscent of immature filopodia. The gene discussed is FMR1; the disease is fragile X syndrome.