FMR1 and fragile X syndrome: The exact nature of the changes to dendritic spine morphology in FXS remains controversial.59 In addition, FXS produces differential effects in the different brain regions, particularly with regard to specific cortical layers.59 In the nucleus accumbens, medium spiny neurons experience NMDA receptor-dependent long-term potentiation that is completely absent in fmr1-KO mice.57 Furthermore, the nucleus accumbens core in fmr1-KO mice is also marked by elongated spines and an increase in filopodial spines on medium spiny neuron spines.57