Figure 1.

Fluorescence resonance energy transfer (FRET). FRET is used for monitoring intra- and inter-molecular interactions occurring on the nanometer scale. The transfer of excitation energy from a “donor” fluorophore (Gag-CFP) to an “acceptor’ fluorophore (Gag-YFP) is monitored. The donor molecule is excited with light at a wavelength that does not excite the acceptor molecule. At distances between donor and acceptor greater than approximately 5 nm, the excited donor molecule emits fluorescence at its characteristic wavelength. At distances below 5 nm, the excited donor molecule can transfer energy to the acceptor molecule, resulting in the acceptor fluorescing at a longer wavelength. In this example, a decrease in Gag-CFP signal, with a corresponding increase in Gag-YFP signal would indicate Gag-Gag interactions.