Cyclic nucleotides are important second messengers in many cellular functions such as visual transduction, and relaxation of smooth muscle cells. Cyclic nucleotides exert their cellular functions through three major classes of cellular receptors, one of them is the cyclic nucleotide-gated (CNG) channels. The CNG channels are non-selective cation channels facilitating the influx of Na+ and Ca2+ ions, following activation by intracellular cyclic nucleotides. In vertebrates, six members of the CNG channel family were identified and grouped according to sequence homology into two subtypes, CNGA and CNGB. To date, four types of the a subunits (CNGA1-4) and two b subunits (CNGB1, CNGB3) have been characterized
