HCN4 and atrial fibrillation: HCN4 gene-related arrhythmias, including atrioventricular block and AF, have still an unexplored role in arrhythmogenesis, although HCN4 gene (cyclic nucleotide gated 4 channel) is generally accepted as a genetic marker in myocardial pacemaker tissues (Difrancesco, 2015).