Although the subunit stoichiometry of the rod CNG channel has been well established, the stoichiometry of the cone CNG channel is still uncertain. Whereas some studies have suggested it also has a 3:1 stoichiometry of CNGA3:CNGB3, others have suggested a 2:2 stoichiometry 8, 10 . Here we find that the CLZ domain of CNGA3 forms a parallel three helix coiled coil that is similar to that of CNGA1. This suggests that the stoichiometry and mechanism of assembly for the cone CNG channel will be similar to that of the rod CNG channel. In addition, it has been shown that other three helix coiled-coil domains can substitute for the CLZ domain in cone CNG channels, but two and four helix coiled-coil domains cannot 20 . Interestingly, the olfactory CNG channel has a stoichiometry of 2:1:1 CNGA2:CNGA4:CNGB1b 9 . Both the CNGA2 and CNGA4 subunits have a CLZ domain (Fig. 1b) . It remains to be determined whether the 2:1:1 stoichiometry comes about from a heterotypic three helix coiled coil of the CNGA2 and CNGA4 subunits, or from other intersubunit interactions such as seen at the level of the C-linker regions in the related hyperpolarization-activated cyclic nucleotide-gated channels 19 . In addition, the 3:1 stoichiometry of the TRPP2/PKD1 complex has recently been proposed to involve a trimeric coiled-coil domain in the carboxy-terminal region of TRPP2 (ref. 42) , suggesting that the assembly mechanism of CNG channels may extend to other distantly related proteins.