HCN4 (Hyperpolarization Activated Cyclic Nucleotide Gated Potassium Channel 4): HCN4 is a vital protein that forms part of the cardiac pacemaker channels responsible for controlling heart rhythm. It plays a key role in generating and regulating the pacemaker current, which influences heart rate and cardiac output. Mutations in HCN4 are linked to various cardiac arrhythmias, such as bradycardia and heart block, making it an important gene for understanding heart function and related disorders.