In conclusion, our results reveal a postsynaptic mechanism by which Fmr1 regulates callosal connectivity that likely contributes to the reduced interhemispheric structural and functional connectivity in Fmr1 KO mice and humans with FXS (Haberl et al., 2015; Swanson et al., 2018; Zerbi et al., 2018). The gene discussed is FMR1; the disease is fragile X syndrome.