Hyperpolarization-activated ion channel with very slow activation and inactivation exhibiting weak selectivity for potassium over sodium ions. Contributes to the native pacemaker currents in heart (If) that regulate the rhythm of heart beat. May contribute to the native pacemaker currents in neurons (Ih). May mediate responses to sour stimuli. Activated by cAMP. cAMP binding causes a conformation change that leads to the assembly of an active tetramer and channel opening. Homotetramer. The potassium channel is composed of a homo- or heterotetrameric complex of pore-forming subunits. Highly expressed in thalamus, testis and in heart, both in ventricle and atrium. Detected at much lower levels in amygdala, substantia nigra, cerebellum and hippocampus. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. The disease is caused by variants affecting the gene represented in this entry. The gene represented in this entry may be involved in disease pathogenesis. Disease susceptibility is associated with variants affecting the gene represented in this entry. Inhibited by extracellular cesium ions. Belongs to the potassium channel HCN family.