Li et al (21) established canine models with age-associated atrial fibrillation or age-associated sinus rhythm, and demonstrated that the If current and HCN4 mRNA expression levels were significantly higher in the age-associated sinus rhythm model, indicating that HCN and the HCN-mediated If, particularly the HCN4 channel current component, may be involved in the pathogenesis of age-associated atrial fibrillation. Here, HCN4 is linked to atrial fibrillation.