HCN4 and Sinus bradycardia: Our data show that mouse models considered in this study are well designed to recapitulate a wide range of hallmarks of human primary SND, from moderate sinus bradycardia (HCN4-CNBD) to SND associated with severe AV block and atrial tachycardia (Cav1.3−/−, Cav1.3−/−/Cav3.1−/−), as well as slowing of AV conduction (Nav1.5+/−).