A possible alternative explanation for the FRET seen between CNGB1 subunits with deletion of the CNGA1 CLZ domain is that the CNGB1 subunits are forming homomeric channels at low levels. Indeed, the fluorescence is considerably dimmer with co-expression of CNGA1-∆CLZ than with CNGA1. Previously, however, it has been shown that, in the absence of CNGA1, CNGB1 subunits do not form functional homomeric channels and are only very weakly expressed in the plasma membrane 9, 17 . Similarly, we found that, in the absence of a CNGA1 subunit, both CNGB1 and CNGB1-∆N did not produce detectable currents and are poorly expressed in the plasma membrane ( Supplementary Fig. S1 ). In addition, it was shown previously that the CNGB1 subunits expressed alone do not exhibit appreciable FRET 9 . This makes it unlikely that the increased FRET seen between CNGB1 subunits with deletion of the CNGA1 CLZ domain is from CNGB1 subunits forming homomeric channels.