BRET technology offers a straightforward genetically encoded approach to ratiometric imaging of PPIs in cells and living subjects. Being a bioluminescence-based technique, which is excitation- and autofluorescence-free, BRET imaging rapidly extended to studies in living subjects. The ratiometric character of BRET imaging eliminates the need of introducing a second reporter for internal referencing, which is normally needed for split luciferases systems. Previous BRET systems were mainly limited to cell culture assays and s.c. tumors because of their green to yellow light emission, which is strongly absorbed by blood in the highly vascularized tissues of living subjects. Owing to their red light emission, the BRET systems presented here expand the scope of BRET techniques to imaging with high sensitivity in deep tissues of small animals. We have paired red-shifted RLuc variants as BRET donors with suitable RFPs (TagRFP or TurboFP635) based on spectral overlapping. The obtained BRET constructs showed high BRET efficiencies in cell culture and provided ratiometric BRET measurements in living mice.