time-resolved fluorescence energy transfer (tr-fret) is the practical combination of time-resolved fluorometry (trf) with förster resonance energy transfer (fret) that offers a powerful tool for drug discovery researchers. tr-fret combines the low background aspect of trf with the homogeneous assay format of fret. the resulting assay provides an increase in flexibility, reliability and sensitivity in addition to higher throughput and fewer false positive/false negative results. fret involves two fluorophores, a donor and an acceptor. excitation of the donor by an energy source (e.g. flash lamp or laser) produces an energy transfer to the acceptor if the two are within a given proximity to each other. the acceptor in turn emits light at its characteristic wavelength.