Previously, it has been shown that the amino-terminal region of the CNGB1 subunit interacts with the CLZ domain of the CNGA1 subunit and regulates trafficking of heteromeric channels 21 . To test for a possible role of this interaction in subunit assembly, we deleted the amino-terminal region (amino acids 2-765) of CNGB1 (CNGB1-∆N). Co-expressing CNGA1 and CNGB1-∆N produced robust currents with amplitude similar to that of CNGA1 + CNGB1 (ref. 21 ). In addition, as shown in Figure 2a , deletion of the aminoterminal region of CNGB1 had no effect on the cAMP efficacy of the heteromeric channels (I cAMP /I cGMP = 0.21 ± 0.03, n = 7). These results suggest that the amino-terminal domain of CNGB1 is not required for the proper assembly of functional heteromeric channels.