To directly measure the presence of multiple CNGB1 subunits in the channel, we turned to an approach based on fluorescence resonance energy transfer (FRET) 23 . FRET reports the proximity of two fluorophores based on the efficiency of energy transfer between a donor fluorophore and an acceptor fluorophore. FRET can be detected by an enhanced emission of the acceptor fluorescence on excitation of the donor. Cyan fluorescent protein (eCFP) and yellow fluorescent protein (eYFP) are genetically-encoded donor and acceptor fluorophores, respectively, that can be fused to proteins of interest 24 . They exhibit 50% energy transfer at a distance of about 50 Å and measurable transfer up to 80 Å (ref. 25) , making them ideally suited for measurements of subunit composition 7 . Previously it was shown that if CNGA1 subunits are fused to eCFP and eYFP (individually) and co-expressed with CNGB1 subunits in Xenopus oocytes, there was significant FRET for channels in the plasma membrane 7 . However, if CNGB1 subunits were fused to eCFP and eYFP and co-expressed with CNGA1 subunits, there was little or no FRET (Fig. 4a) . These results revealed that heteromeric rod CNG channels in the plasma membrane contain multiple CNGA1 subunits but only a single CNGB1 subunit (3:1 stoichiometry).