Norak Biosciences, Inc. is using its proprietary Transfluor?technology as a universal, cell-based, imaging assay for known and orphan GPCRs. The Transfluor?technology is based on a physiological process common to nearly all GPCRs--receptor desensitization. Specifically, the assay utilizes the redistribution of fluorescently-labeled arrestins from the cytoplasm to receptors at the plasma membrane to monitor the activation (or inactivation) of GPCRs. Transfluor?is quantitated on multiple automated image analysis systems, achieving high signal to background ratios (5:1 to 25:1) and screening rates of 50,000 compounds per day. The assay discriminates between agonists, partial agonists, and antagonists while providing valuable pharmacological information on efficacy and potency. In contrast to present methods of screening GPCRs, the power of the Transfluor?assay is in its simplicity, sensitivity, and applicability to all GPCRs without requiring prior knowledge of natural ligands or how a given receptor is coupled to downstream signaling pathways.