Bioluminescence-based methods hold particular promise for imaging of PPIs in small living subjects (12, 13). Although both BRET and FRET detection schemes rely on the Förster resonance energy transfer mechanism (14), BRET systems provide enhanced sensitivity in living subjects, because autofluorescence, photobleaching, and tissue attenuation associated with fluorophore excitation are absent. Split luciferase complementation systems are advanced designs for in vivo investigation of PPIs. The luciferase fragments are nonfunctional in the absence of protein association; when the proteins interact, the luciferase regains its reporter function and generates a bioluminescent turn-on signal. However, BRET probes could potentially offer several advantages over split luciferase for interrogating PPIs in living subjects. First, the higher light output of BRET translates into enhanced sensitivity for small animal imaging; generally, split luciferases after complementation exhibit only 20–50% of the activity of corresponding intact luciferase (15). Second, BRET enables real-time two-color ratiometric measurements. Third, false-negative signals associated with misfolding of the reconstituted protein and false-positive signals arising from nonspecific association of the split fragments are significantly reduced for BRET relative to split protein reporters.