The results from the above experiments, and those of others, suggest a molecular mechanism for the controlled assembly of rod CNG channels with a 3:1 stoichiometry of CNGA1:CNGB1 subunits (Fig. 8a) [6] [7] [8] [9] 20 . In this mechanism, the CNGA1 subunits initially assemble into trimers through homotypic association of their carboxy-terminal CLZ domains into a parallel three helix coiled coil. Subsequently, a CNGB1 subunit, if present, is preferentially incorporated into the remaining slot of the channel tetramer because of a high affinity association with CNGA1 subunits. This mechanism incorporates aspects of two previous models for heteromeric assembly of CNG channels: initial assembly of CNGA1 trimers 8 , and high affinity association between CNGA1 and CNGB1 subunits 9,21 . The trimerization of the CLZ domain imposes the constraint that the channels contain at least three CNGA1 subunits, whereas the high affinity association between CNGA1 and CNGB1 assures that the fourth subunit is CNGB1. In support of this mechanism, we showed with electrophysiology and fluorescence that deletion of the CLZ domain in CNGA1 causes multiple CNGB1 subunits to incorporate into the heteromeric CNG channel. Furthermore, with X-ray crystallography we showed that there is a trimerization of the CLZ domain for both CNGA1 and CNGA3 (Fig. 5) that results from the formation of a parallel three helix coiled coil (Fig. 6) .