bioluminescence bioluminescence resonance energy transfer BRET BRET is similar to FRET but has a bioluminescent protein such as aequorin as the donor and a fluorescent acceptor, e.g. GFP. The donor emits blue light in the absence of GFP, but when GFP and aequorin are associated as they are in vivo, GFP accepts the energy from aequorin and emits green light. Hence BRET eliminates the disadvantages associated with donor fluorophore activation.