Hyperpolarization- and Cyclic Nucleotide-gated (HCN) channels are a family of six transmembrane domain, single pore-loop, hyperpolarization activated, non-selective cation channels. The HCN family consists of four members (HCN1-4). HCN channels represent the molecular correlates of I(h), a hyperpolarization-activated current best known for its role in controlling heart rate and in the regulation of neuronal resting membrane potential and excitability.

HCN channels are unique among vertebrate voltage-gated ion channels, in that they have a reverse voltage-dependence that leads to activation upon hyperpolarization. HCN channels are encoded by four genes (HCN1-4) and are widely expressed throughout the heart and the central nervous system.