Hyperpolarization-activated ion channel exhibiting weak selectivity for potassium over sodium ions (PubMed:28086084). Contributes to the native pacemaker currents in heart (If) and in neurons (Ih). May mediate responses to sour stimuli. Activated by cAMP, and at 10-100 times higher concentrations, also by cGMP. cAMP binding promotes tetramerization and formation of an active channel. Compared to other family members, cAMP has less stimulatory effect on HCN1 because part of the molecules already contain bound cAMP and form homotetramers when cAMP levels are low. Inhibited by Cs(1+), zatebradine, capsazepine and ZD7288. Homotetramer (PubMed:28086084). Heterotetramer with HCN2. The potassium channel is composed of a homo- or heterotetrameric complex of pore-forming subunits. Interacts with KCNE2. Interacts with the SH3 domain of CSK (By similarity). Detected in brain, in particular in amygdala and hippocampus, while expression in caudate nucleus, corpus callosum, substantia nigra, subthalamic nucleus and thalamus is very low or not detectable. Detected at very low levels in muscle and pancreas. 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 disease is caused by variants affecting the gene represented in this entry. Belongs to the potassium channel HCN family. Contaminating sequence. Potential poly-A sequence.