How does the complex subunit composition of CNG channels come about? The CNGA subunits contain a carboxy-terminal leucine zipper (CLZ) domain in the post-CNBD region (Fig. 1a,b) 8 . On the basis of the finding that the CLZ domain from CNGA3 forms homotypic trimers in solution, it was proposed that trimerization of this domain constrains the channel to contain three CNGA subunits 8 . Previously, it has been shown that point mutations in this domain decreased the proportion of heteromeric channels (relative to CNGA homomers) but did not alter the 3:1 stoichiometry of the heteromeric channels 20 .