Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are low-threshold, voltage-gated ion channels, that are normally activated at negative potentials. Intracellular cAMP shifts the voltage dependence of these channels to less hyperpolarized potentials. Ih encode several ion channel isoforms with long or short N-termini and variations in the length of the interloop regions between the membrane-spanning domains S3-S4 and S4-S5. (Adapted from FBrf0227998 and FBrf0184031).