HCN4 and cardiac arrhythmia: Others previously showed that HCN4+/− humans are typically characterized by bradycardia44; Hcn4−/− mice die prenatally, without arrhythmias and with lower heart rates when compared with Hcn4+/− and Hcn4+/+ mice36; and 2dpf zebrafish embryos with experimentally downregulated hcn4 expression have a higher odds of sinoatrial arrests and lower heart rate compared with un-injected controls15.