To unravel the cause of this intermediate block, we measured the dose-response relations for L-cis-diltiazem block at + 60 mV. The dose-response relations for channels containing CLZ domains were well fit with a single Langmuir isotherm, suggesting a single population of channels (Fig. 3b) . The block of CNGA1 and CNGA1-∆CLZ homomers were similar and low affinity (K 1/2 = 174 ± 7.2 µM, n = 5 and K 1/2 = 164 ± 8.2 µM, n = 6, respectively), and the block of CNGA1 + CNGB1 and CNGA1 + CNGB1-∆N heteromers were similar and high affinity (K 1/2 = 2.45 ± 0.28 µM, n = 5 and K 1/2 = 1.40 ± 0.21 µM, n = 5, respectively) (Fig. 3b) . The dose-response relation for block of CNGA1-∆CLZ + CNGB1-∆N heteromers, however, was not fit well by either a single Langmuir isotherm, nor by the sum of two Langmuir isotherms with affinities constrained to those of CNGA1 homomers and CNGA1 + CNGB1 heteromers. Instead, fits to the sum of two Langmuir isotherms revealed a small high affinity component similar to that of CNGA1 + CNGB1 channels (K 1/2 = 0.56 ± 0.09 µM, n = 4), and a larger component with an affinity intermediate between that of CNGA1 homomers and CNGA1 + CNGB1 heteromers (K 1/2 = 50 ± 5 µM, n = 4) (Fig. 3b) . These results suggest that the heteromers formed from CNGA1-∆CLZ and CNGB1-∆N are not a mixture of CNGA1 homomers and normal CNGA1 + CNGB1 heteromers, but, instead, a mixture of normal 3:1 CNGA1 + CNGB1 heteromers and novel CNGA1 + CNGB1 heteromers.