Here we show that both control and FMRP-KO hiPSCs can be converted to functionally mature cortical neurons and cerebral organoids, and that these in vitro systems display, up to 100 days in vitro, a FXS phenotype and can be considered promising disease models to study FXS and to develop and test disease-modifying drugs. This evidence concerns the gene FMR1 and fragile X syndrome.