HCN4 (Hyperpolarisation 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 key role in generating and controlling the pacemaker current, which affects heart rate and cardiac output. Mutations in HCN4 are associated with various cardiac arrhythmias, such as bradycardia and heart block, making it an important gene for understanding heart function and related disorders.