Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in the heart and in the nervous system. Hyperpolarization-activated cation currents produced by HCN channels (Ih, If or Iq), contribute to a wide range of physiological functions including cardiac and neuronal pacemaker activity, the setting of resting potentials, dendritic integration of synaptic transmission and learning.