FMR1 and fragile X syndrome: Indeed, rodent models of FXS have reported that some classes of principal neurons lacking FMRP display altered intrinsic excitability [9, 12–14, 36] although others have not observed difference in these parameters [15, 37] most likely reflecting the fact that where differences do occur these are cell-type specific, rather than a generalized alteration in intrinsic excitability due to the loss of expression of FMRP.