One intriguing possibility is that the novel heteromers formed without a CLZ domain contain multiple CNGB1 subunits. This would also suggest that whereas one CNGB1 subunit confers much greater L-cis-diltiazem affinity, extra CNGB1 subunits lower the L-cis-diltiazem affinity (Fig. 3d) . This model predicts that lowering the relative amount of expressed CNGB1 subunits should actually increase the fraction of the high affinity component (due presumably to channels with the normal 3:1 CNGA1:CNGB1 stoichiometry), because a lower amount of CNGB1 should favour a subunit stoichiometry with fewer CNGB1 subunits. The results of such an experiment are shown in Figure 3c . Lowering the RNA co-injection ratio of CNGA1-∆CLZ and CNGB1-∆N from 1:2 to 1:1 significantly increased the fraction of the high affinity component from 0.21 ± 0.01, n = 3 to 0.40 ± 0.01, n = 4. In addition, the dose-response relation is now poorly fit by a sum of two Langmuir isotherms and is suggestive of a third component with an affinity similar to CNGA1 homomers. Together, these results suggest that deletion of the CLZ domain removes the constraint allowing only one CNGB1 subunit in the channel tetramer. This allows multiple CNGB1 subunits that confer higher efficacy for cAMP and (surprisingly) lower affinity for L-cis-diltiazem block (Fig. 3d ). If channels containing three CNGB1 subunits were not functional or not trafficked properly to the plasma membrane, then this loss of controlled assembly could also explain the large dominant negative effect of CNGB1 subunits, when the CLZ domain is deleted from the CNGA1 subunit.