Fluorescein di(β-D-galactopyranoside) (FDG) is widely used as a substrate in enzymatic assays, particularly for detecting β-galactosidase activity. It is commonly applied in molecular biology and biochemistry research to study gene expression and cell labeling. When FDG is cleaved by β-galactosidase, it releases fluorescein, which emits a strong fluorescent signal. This property makes it valuable for flow cytometry, fluorescence microscopy, and high-throughput screening to identify and quantify β-galactosidase-producing cells. Additionally, FDG is utilized in microbiology to detect bacterial colonies expressing β-galactosidase, aiding in genetic engineering and diagnostic applications. Its sensitivity and specificity make it a crucial tool in both research and clinical settings.