The PhosphoSeek™ Technology for screening of phosphodiesterase activities is a robust and homogeneous detection platform that measures the activity of a target enzyme. Assays are non-competitive with respect to substrate and do not require radioactive materials or secondary (detector) enzymes or antibodies. Assays can be performed using defined reaction components (biochemical assay) or for detection of endogenous enzyme activities using cellular lysates. Enzyme progress can be monitored after reactions have occurred or in continuous mode as substrate conversion occurs in real time. The assays are ideally suited for automated screening and can be read on any fluorometer. The Sensor is a proprietary molecule that contains a trivalent metal ion, which binds to phosphoryl groups on biological substrates. Activity is measured by the change of fluorescence of a dye-labeled cyclic guanosine monophosphate (cGMP) upon hydrolysis and binding by Sensor. The change in fluorescence directly correlates to the level of substrate conversion.