selective than others. Even though the activity of these channels show little voltage dependence, they are still considered voltage-dependent channels. Calcium, calmodulin, and phosphorylation modulate the opening of CNG channels. The main role of CNG channels is sensory transduction in various tissues. Many studies have shown CNG channels in rod and cone photoreceptors, and they have also been found in the brain, the heart, the kidneys, and gonads. CNG channel homologs in Caenorhabditis elegans, Drosophila melanogaster, and Limulus polyphemus have unknown functions. Studies have shown homologs in C. elegans might have functions in chemosensation. Color blindness and retinal degeneration result when CNG channels have mutations. Mutations specifically in the subunits A and B result in complete and incomplete achromatopsia. Structure A CNG channel consists of four subunits around a central pore. Each protein subunit consists 6 transmembrane segments (S1-S6), a P-loop, intracellular amino terminal region, and carboxy terminal region. The P-loop and S6 segments around the pore, which plays a role in ion conduction. There is a cyclic nucleotide binding domain (CNBD) and connection region to the S6 segment in the carboxy terminal. There is a post-CNDB region in the amino terminal. Alpha subunits Cyclic nucleotide gated channel alpha-subunits include Cyclic nucleotide-gated channel alpha 1 Cyclic nucleotide-gated channel alpha 2 Cyclic nucleotide-gated channel alpha 3 Cyclic nucleotide-gated channel alpha 4 Beta subunits Cyclic nucleotide gated channel beta-subunits include: Cyclic nucleotide-gated channel beta 1 Cyclic nucleotide-gated channel beta 3 Pore The structure of the pore is similar to other ion channels that contain P-loops. The P-loop enters the membrane of the pore from the extracellular side and exits to the intracellular side. The P loop enters as an alpha helix and exists as an uncoiled strand. Helices that cover the inner membrane line the channel. These also form a 6 helix bundle that signifies the entrance. In order to open the pore, a conformational change must occur in the inner 6 helix bundle. Cyclic nucleotide binding domain A Cyclic nucleotide-binding domain is an intracellular domain located in the C-terminus region and has a similar sequence to other cyclic nucleotide-binding proteins. The domain is believed to be made up of a β-pleated sheet and two α-helices. The β-pleated sheet is made up of eight antiparallel strands. The α helices are named the B and C helices. A ligand initially binds to the β-pleated sheet, and through allosteric regulation causes the movement to