FMR1 and fragile X syndrome: One example is delayed critical period for GABA polarity in the FXS mouse model (also known as Fmr1 deficient mice; He et al., 2014) and in mouse models of Rett syndrome (Mecp2 deficient mice) displaying abnormal GABA polarity in the hippocampus (excitatory GABA instead of inhibitory), until very late in brain development (i.e., P15 in mice (Lozovaya et al., 2019).