Hyperpolarization-activated potassium channel. May also facilitate the permeation of sodium ions. Inhibited by Cs(1+) and ivabradine. Is apparently not activated by cAMP or cGMP. Homotetramer. The potassium channel is probably composed of a homo- or heterotetrameric complex of pore-forming subunits (By similarity). Interacts with KCTD3 and PEX5L (PubMed:23382386). Detected in hypothalamus, amygdala, olfactory bulb, hippocampus and retina (at protein level). Highly expressed in brain and heart, in particular in ventricle, atrium and in sinoatrial node (SAN). Detected at low levels in skeletal muscle and lung. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Belongs to the potassium channel HCN family.