Standard BRET systems consist of a luciferase, which in the presence of its bioluminogenic substrate, acts as a resonance energy donor, and a fluorescent protein, which is the resonance energy acceptor. To successfully translate a BRET assay from cell culture to living subjects, it is critical that the BRET system is characterized by efficient energy transfer, excellent spectral resolution, a BRET donor with high bioluminescence quantum yield, and a red light-emitting BRET acceptor. Current BRET systems emit light mostly in the green to yellow region of the visible spectrum (510–570 nm), rendering them suboptimal for imaging in living subjects. Examples of such systems include BRET1 (16) and BRET2 (17), which consist of the donor Renilla reniformis luciferase [RLuc; λem = 480 nm for coelenterazine (CLZ) and λem = 395 nm for DeepBlueC] paired with either YFP (λex/λem = 514/530 nm) or GFP (λex/λem = 400/510 nm), respectively. Firefly luciferase (λem = 565 nm) combined with the red fluorescent protein (RFP) DsRed (λex/λem = 558/583 nm) provides a red-shifted signal; however, this system lacks sensitivity and spectral resolution (18, 19).