Used as a chromogenic substrate in enzymatic assays to detect β-glucuronidase activity. The compound is colorless in solution until hydrolyzed by β-glucuronidase, which cleaves the glucuronide bond and releases phenolphthalein. In alkaline conditions, the released phenolphthalein turns pink to red, providing a visual or spectrophotometric signal. This reaction is widely employed in clinical diagnostics, microbiology, and molecular biology for identifying bacterial species such as Escherichia coli, which produce β-glucuronidase. It is also used in cell-based assays and reporter gene systems where β-glucuronidase serves as a marker enzyme.