Their very different ability to block a potential charge compensating current through RyR2 means that intracellular QX‐FL is a powerful tool for establishing the relative contribution of Na+ channel block, and a potential action on RyR2 channels, to flecainide's inhibition of inappropriate RyR2 channel‐mediated Ca2+ release in CPVT. The gene discussed is RYR2; the disease is catecholaminergic polymorphic ventricular tachycardia.