sentences:  The molecular determinants of the pacemaker current belong to the HCN channel (hyperpolarization-activated cyclic nucleotide–gated channel), of which 4 isoforms (HCN1 to HCN4) are known.Based on their sequence, HCN channels are classified as members of the superfamily of voltage-gated K+ (Kv) and CNG channels.Because of their relevance to generation of pacemaker activity and modulation of spontaneous frequency, f-channels are natural targets of drugs aimed to pharmacologically control heart rate.