HCN4 encodes a hyperpolarization-activated ion channel permeable to both sodium and potassium ions, with higher selectivity for potassium. The channel exhibits very slow activation and inactivation kinetics and is a primary component of the funny current (If) that drives pacemaker activity in the sinoatrial node and regulates cardiac rhythm. HCN4 is also expressed in the atrioventricular node, where it contributes to conduction properties, and in neuronal tissue where it generates the analogous Ih current 1. cAMP and protein kinase A serve as direct regulators of HCN4 function. HCN4 variants are implicated in multiple cardiac arrhythmias. Transmembrane variants in HCN4 are enriched in left ventricular noncompaction cases, supporting associations with arrhythmogenic phenotypes 2. The G1097W variant has been clinically linked to atrioventricular block 1. In heart failure and atrial fibrillation, reduced HCN4 expression and down-regulation of the If current contribute to sinus node remodeling 3. Recent studies demonstrate that eliminating HCN4 expression in stem cell-derived cardiomyocytes, combined with modulation of related ion channels, prevents automaticity and engraftment arrhythmias in transplantation models 4. Clinically, ivabradine, an HCN4 inhibitor, is approved for heart rate reduction; additional therapeutic options include dronedarone for arrhythmia management 1. Emerging evidence suggests HCN channels may contribute to absence seizures, though HCN4's specific role remains less characterized than HCN1 and HCN2 in this context 5.