A limitation of FRET performed with fluorophore donors is the requirement for external illumination to initiate the fluorescence transfer, which can lead to background noise in the results from direct excitation of the acceptor or to photobleaching. To avoid this drawback, bioluminescence resonance energy transfer (or BRET) has been developed. This technique uses a bioluminescent luciferase (typically the luciferase from Renilla reniformis) rather than CFP to produce an initial photon emission compatible with YFP.
BRET has also been implemented using a different luciferase enzyme, engineered from the deep-sea shrimp Oplophorus gracilirostris. This luciferase is smaller (19 kD) and brighter than the more commonly used luciferase from Renilla reniformis, and has been named NanoLuc or NanoKAZ. Promega has developed a patented substrate for NanoLuc called furimazine, though other valuables coelenterazine substrates for NanoLuc have also been published. A split-protein version of NanoLuc developed by Promega has also been used as a BRET donor in experiments measuring protein-protein interactions.