HCN4 and cardiac arrhythmia: HCN4 channels mediate pacemaker activity in the heart (Brown et al., 1979) and many mutations associated with cardiac arrhythmias have been characterized (Schulze-Bahr et al., 2003; Milanesi et al., 2006; Baruscotti et al., 2010; DiFrancesco, 2013, 2015).