Cyclic nucleotide–gated ion channels or CNG channels are ion channels that function in response to the binding of cyclic nucleotides. CNG channels are nonselective cation channels that are found in the membranes of various tissue and cell types, and are significant in sensory transduction as well as cellular development. Their function can be the result of a combination of the binding of cyclic nucleotides (cGMP and cAMP) and either a depolarization or a hyperpolarization event. Initially discovered in the cells that make up the retina of the eye, CNG channels have been found in many different cell types across both the animal and the plant kingdoms. CNG channels have a very complex structure with various subunits and domains that play a critical role in their function. CNG channels are significant in the function of various sensory pathways including vision and olfaction, as well as in other key cellular functions such as hormone release and chemotaxis. CNG channels have also been found to exist in prokaryotes, including many spirochaeta, though their precise role in bacterial physiology remains unknown. Discovery The discovery of CNG channels is related to the discovery of intracellular messengers responsible for the mediation of responses in retinal photoreceptors. Before their discovery, it was thought that cyclic nucleotides played a role in phosphorylation. In 1985, it was discovered that cGMP was able to directly activate the light-dependent response of rod ion channels by studying light-adapted retina of frogs. CNG channels were also found in cone photoreceptors, chemo sensitive cilia of olfactory sensory neurons, and the pineal gland. After the identification of amino acids from purified proteins, cloning and functional expression of CNG channels were performed. Molecular cloning allowed for the discovery of similar channels in many other tissues. In 2000, scientists performed studies using mouse retina and molecular cloning to find a new subunit of the channel, CNG6. Function CNG channels have important functions in signal transduction in retinal photoreceptors and olfactory receptor neurons. They are directly activated by cyclic nucleotides, and approximately 4 cyclic nucleotides are needed to activate each channel. CNG channels are nonselective and allow many alkali ions to flow into or out of a cell expressing CNG channels on its membrane. This flow of ions can result in either depolarization or hyperpolarization. CNG channels can be activated by cAMP or cGMP exclusively, or sometimes by a combination of both cNMPs, and some channels are more