Cyclic nucleotide-gated (CNG) channels, however, have a more complex subunit composition. Native CNG channels are composed of a stereotyped number of CNGA subunits and CNGB subunits. CNGA and CNGB subunits are structurally related and both are thought to contribute to the formation of the pore. The CNG channel from rod photoreceptors is composed of three CNGA1 subunits and one CNGB1 subunit [6] [7] [8] , whereas the CNG channel from olfactory receptors is composed of two CNGA2 subunits, one CNGA4 subunit and one CNGB1b subunit 9 . The cone CNG channels comprise CNGA3 subunits and CNGB3 subunits, though the subunit stoichiometry is still unknown 8, 10 . The presence of the CNGB subunits confers enhanced Ca 2 + permeation, modulation by Ca 2 +calmodulin, and native-like cyclic-nucleotide specificity compared with homomeric CNGA channels [11] [12] [13] [14] [15] [16] [17] . The precisely controlled subunit compositions, therefore, tune the CNG channels for their specialized roles in phototransduction and olfactory transduction.