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Mutations in <i>SCN1A</i> gene, encoding the voltage-gated sodium channel (VGSC) Na_V1.1, are widely recognized as a leading cause of genetic febrile seizures (FS), due to the decrease in the Na⁺ current density, mainly affecting the inhibitory neuronal transmission. Here, we generated induced pluripotent stem cells (iPSCs)-derived neurons (idNs) from a patient belonging to a genetically well-characterized Italian family, carrying the c.434T > C mutation in <i>SCN1A</i> gene (hereafter SCN1A^M145T). A side-by-side comparison of diseased and healthy idNs revealed an overall maturation delay of SCN1A^M145T cells. Membranes isolated from both diseased and control idNs were injected into <i>Xenopus</i> oocytes and both GABA and AMPA currents were successfully recorded. Patch-clamp measurements on idNs revealed depolarized action potential for SCN1A^M145T, suggesting a reduced excitability. Expression analyses of VGSCs and chloride co-transporters <i>NKCC1</i> and <i>KCC2</i> showed a cellular “dysmaturity” of mutated idNs, strengthened by the high expression of SCN3A, a more fetal-like VGSC isoform, and a high <i>NKCC1</i>/<i>KCC2</i> ratio, in mutated cells. Overall, we provide strong evidence for an intrinsic cellular immaturity, underscoring the role of mutant Na_V1.1 in the development of FS. Furthermore, our data are strengthening previous findings obtained using transfected cells and recordings on human slices, demonstrating that diseased idNs represent a powerful tool for personalized therapy and ex vivo drug screening for human epileptic disorders.