Representative results from co-expression of CNGA1-∆CLZ with eCFP-CNGB1-∆N and eYFP-CNGB1-∆N are shown in Figure 4b . FRET was measured from the sensitized emission of the acceptor using fluorescence spectra 23 . The fluorescence emission spectra from channels in or near the plasma membrane of live Xenopus oocytes were measured with confocal microscopy. Excitation of eCFP with a 458 nm laser produced a complex emission spectrum (red trace) with both eCFP and eYFP emission components. The eYFP component (orange trace) was extracted by subtracting off a scaled emission spectrum from eCFP alone (blue trace). This extracted spectrum contains eYFP emission from both FRET and direct excitation of eYFP by 458 nm light. To isolate the component due to FRET, the fractional excitation of eYFP by 458 nm light relative to 488 nm light (Ratio Ao) (Fig. 4c, Methods) was subtracted off the corresponding fractional excitation for eCFP and eYFP-containing channels (Ratio A). Ratio A-Ratio Ao is directly proportional to the FRET efficiency 23 . As shown in Figure 4d , Ratio A-Ratio Ao was significantly larger with CNGA1-∆CLZ than with CNGA1, when the CNGA1 subunit was coexpressed with fluorescent CNGB1. These results directly demonstrate that deletion of the CLZ domain caused multiple CNGB1 subunits to assemble.