To further test for incorporation of multiple CNGB1 subunits on deletion of the CLZ domain, we measured the pharmacological properties of heteromeric channels. Although 100 µM L-cisdiltiazem is ineffective at blocking CNGA1 homomers, it potently blocks CNGA1 + CNGB1 heteromers (Fig. 3a , black is control, blue is 100 µM L-cis-diltiazem) 17 . The block is voltage dependent with a Hill slope of one, suggesting a pore-blocking mechanism (Fig. 3a,b) . The CNGA1-∆CLZ mutation had no effect on block of homomeric channels, and the CNGB1-∆N mutation had no effect on block of heteromeric channels. Surprisingly, the block of CNGA1-∆CLZ + CNGB1-∆N heteromers was intermediate between CNGA1 (or CNGA1-∆CLZ) homomers and , and CnGA1-∆CLZ + CnGB1-∆n (n = 10). The expected subunit stoichiometry for each case is shown at the bottom. Data are plotted as mean ± s.e.m. Asterisks denote statistically significant differences with P < 0.01. CNGA1 + CNGB1 (or CNGA1 + CNGB1-∆N) heteromers (Fig. 3a) . Previously, it was shown that heteromers formed with point mutations in the CLZ domain of CNGA3 exhibited intermediate block by L-cis-diltiazem because they formed mixtures of CNGA3 homomers and CNGA3 + CNGB1 heteromers 20 . However, that was not the case here. As shown above, CNGA1-∆CLZ + CNGB1-∆N heteromers actually exhibit greater cAMP efficacy than either CNGA1 homomers or CNGA1 + CNGB1 heteromers (Fig. 2) . Together, these results indicate the formation of a previously uncharacterized population of heteromeric CNG channel complexes.