||Fluorescein diphosphate (FDP) is one of the most sensitive fluorogenic phosphatase substrates commercially available. The colorless and nonfluorescent FDP is hydrolyzed to highly fluorescent fluorescein, which exhibits excellent spectral properties that match the optimal detection window of most fluorescence instruments. Alternatively, FDP can also be used to detect phosphatases in a chromogenic mode since the enzymatic product (fluorescein) exhibits a large extinction coefficient (close to 100,000 cm-1mol-1). FDP has been widely used in various ELISA assays. Additionally it is also used to detect tyrosine phosphatases. FDP is not very thermally stable, and special cautions need be excised for storing the solid sample and stock solutions.