Bioluminescence resonance energy transfer has developed in recent years as a new technique to study protein-protein interactions. Protein partners of interest are tagged with either luciferase or green fluorescent protein (GFP). Non-radiative energy transfer between the excited luciferase and the GFP permits the study of spatial relationships between the two partners. This technique constitutes an important tool for the study of the functional activity of different types of receptors, and can be used in sensitive, homogenous high-throughput screening assays.