TLX2 and hydrops fetalis: The intracellular Na+ concentration increases in HF and the action potential prolongs and so overexpression of the NCX protein on the one hand may aid Ca2+ efflux but, on the other, may support more Ca2+ influx, adding to the increased diastolic Ca2+ concentration caused by poorer SERCA2a uptake and enhanced RyR leak (Fig. 3).