HCN4 (Hyperpolarization Activated Cyclic Nucleotide Gated Potassium Channel 4): HCN4 is an essential protein that forms part of the cardiac pacemaker channels responsible for regulating heart rhythm. It plays a crucial role in generating and controlling the pacemaker current, which affects heart rate and cardiac output. Mutations in HCN4 are associated with various cardiac arrhythmias, including bradycardia and heart block, making it a significant gene for understanding heart function and related disorders.