A receptacle connector includes a housing having a mating end for mating with a plug connector and a mounting end for mounting to a circuit board. The housing has side walls and end walls with contacts in contacts arrays along both side walls. The contacts have mating ends and terminating ends. The housing has a card slot open at the mating end for receiving plug connector defined by interior surfaces of the side walls. The mating ends of the contacts are exposed in the card slot for mating electrical connection with the plug connector. The housing has alignment tabs extending inward into the card slot from the interior surface for aligning the plug connector within the card slot.