As with cortical postsynaptic dendritic spine morphogenesis defects in FXS patient brain autopsies [74, 75], FMR1 KO mice exhibit an immature dendritic spine profile with more long, thin filopodia-like spines and a proportional lack of mature short, stubby, mushroom-like spines [76–79]. The gene discussed is FMR1; the disease is fragile X syndrome.