Potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 4 is a protein that in humans is encoded by the HCN4 gene. There are four HCN channels. HCN4 is prominently expressed in the pace maker region of the mammalian heart. Some humans with bradycardia and Sick sinus syndrome have been shown to have mutations in their HCN4 gene. The role of HCN channels in autonomic control of heart rate is currently a matter of ongoing investigation.