Given E/I balance is established during the critical period, FMRP loss during this developmental window consistently causes differential activity regulation of excitatory vs. inhibitory neurons in the Drosophila FXS model, with defective activity-dependent synapse morphogenesis and Ca2+ signaling maturation (Doll and Broadie, 2015, 2016). The gene discussed is FMR1; the disease is fragile X syndrome.