HCN4 (Hyperpolarization Activated Cyclic Nucleotide Gated Potassium Channel 4): HCN4 is a crucial protein that is part of the cardiac pacemaker channels responsible for controlling heart rhythm. It plays a key role in generating and regulating 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.