ion selectivity of the cyclic-nucleotide gated ion channel, which also determine the pore diameter of CNG channels. The P region functions as a channel gate since it prevents ion permeation in the closed state. The pore may be hindered by small conformational changes in this region. The P region acts as an ion selectivity filter that changes structure in the open conformation. In the open state, four identical subunits contribute a single P-loop region, which forms a selectivity filter. CNG channel family In vertebrates, the CNG channel gene family consists of six members. These genes are divided based on sequence similarity into two subtypes CNGA and CNGB. Additional genes that code for CNG channels have been cloned from Caenorhabditis elegans and Drosophila melanogaster. A subunit of a CNG channel CNGA1, previously called the rod α subunit, was expressed in rod photoreceptors and produced functional channels that were gated by cGMP when expressed externally in either Xenopus oocytes or in a human embryonic kidney cell line (HEK293). In humans, mutated CNGA1 genes result in an autosomal recessive form of retinitis pigmentosa, a degenerative form of blindness. CNGB1, previously called the rod β subunit, is a second subunit of the rod channel. Unlike CNGA1, CNGB1 subunits expressed alone do not produce functional CNG channels, but coexpression of CNGA1 and CNGB1 subunits produces heteromeric channels with modulation, permeation, pharmacology, and cyclic-nucleotide specificity comparable to that of native channels. CNG channels form tetramers, and recent studies indicate that native rod channels consist of three CNGA1 subunits and one CNGB1 subunit. CNGA3 subunits, previously called the cone α subunits, form functional channels when its expression occurs exogenously. On the other hand, CNGB3, previously called the cone β subunit, does not. Mutations in human CNGA3 and CNGB3 are involved in complete achromatopsia, which is a rare, autosomal recessive inherited and congenital disorder characterized by the complete failure in color distinction. CNGA2, previously called the olfactory α subunit, CNGA4, previously called the olfactory β subunit, and CnGB1b are involved in transduction of odorant signals in olfactory sensory neurons for which the subunit stoichiometry and arrangement are unknown. In invertebrates, a CNG channel subunit called CNG-P1 has been cloned from D. melanogaster and is expressed in antennae and the visual system, an indication that CNG channels may be linked to the transduction of light in invertebrates. A second putative CNG-like subunit called CNGL, cloned from D. melanogaster, is