FMR1 and fragile X syndrome: These considerations prompted us to examine possible E/I ratio dysregulation in two ASD-related mouse models with disparate genetic causes: the Fmr1 knockout (Fmr1 KO) mouse, which models Fragile X syndrome,23 the most frequent monogenic cause of intellectual disability and ASD in humans,24 and the BTBR T + Itpr3tf/J (BTBR) mouse, an inbred line that models idiopathic ASD.25