Notably, genes KCNJ5, KCNN2 and HCN4, encoding the inward rectifier K+-channel subunit Kir3.4 (GIRK4), the small conductance calcium activated potassium channel member 2 KCa2.2 (SK2) and the hyperpolarization activated cyclic nucleotide gated potassium channel 4 (HCN4), respectively, were down-regulated in AF. The gene discussed is HCN4; the disease is atrial fibrillation.