HCN2 and atrial fibrillation: Similarly, the aged atrial fibrillation group exhibited significantly enhanced HCN2 and HCN4 protein expression levels compared to those of the aged sinus rhythm group (P<0.05; HCN2, 1.04±0.19 vs. 0.92±0.12; HCN4, 1.12±0.11 vs. 1.02±0.08; Fig. 2).