HCN4 and atrial fibrillation: Compared with the aged sinus rhythm group, the aged atrial fibrillation group also exhibited significantly enhanced levels of HCN2 and HCN4 mRNA expression (P<0.05; HCN2, 1.00±0.08 vs. 0.49±0.07; HCN4, 1.05±0.23 vs. 0.53±0.09; Fig. 1).