CXCR4 and CXCR7 form homodimers and heterodimers in living HEK-293T cells. (A-D) BRET titration curves were generated in cells expressing a constant amount of either CXCR4-Rluc (A,C) or CXCR7-Rluc (B,D) and increasing quantities of YFP or YFP-tagged receptors, stimulated (open symbols) or not (closed symbols) with 1 μM CXCL12. BRET₅₀ values presented in the tables were deduced from data analysis using a nonlinear regression equation applied to a single binding site model and are representative of 3 to 5 independent experiments. BRET_max signals from cells containing CXCR4 or CXCR7 homodimers or heterodimers were significantly increased by CXCL12 compared with basal conditions (P < .05).