Naphthol AS-TR Phosphate is a chromogenic substrate primarily used in histochemical and biochemical assays for detecting alkaline phosphatase (ALP) and acid phosphatase activities. It serves as a phosphate ester that, upon enzymatic hydrolysis, releases the naphthol AS-TR coupler. This coupler then rapidly reacts with a diazonium salt, such as Fast Red TR or Fast Blue BBN, to form an insoluble, brightly colored azo dye precipitate (typically red to violet) at the exact site of enzyme localization. This method enables precise visualization of phosphatase distribution in tissue sections, cells, and biological samples. It is essential in immunohistochemistry, enzyme histochemistry, diagnostic pathology for conditions like liver disease, bone disorders, and malignancies, as well as in research and quality control applications. The substrate offers high sensitivity, specificity, and stability, making it a standard reagent in clinical and laboratory settings.