Our findings map the developmental window of NMD hyperactivation in the Fmr1-KO mouse model of FXS and provide a foundation to further examine the effect of hyperactivated NMD on neuronal differentiation in the cortex, hippocampus, and cerebellum, as well as a rationale for early intervention in FXS using NMD inhibitors. Here, PLA1A is linked to fragile X syndrome.