Green fluorescent thiol-reactive dye (Ex/Em: 494/519nm) for bioconjugation, widely used as protein and peptide fluorescent labeling reagent, easily conjugating fluorescein. The reaction requires very mild condition and is highly specific. The maleimide group reacts optimally with sulfhydryl groups on cysteine side chains at pH 7, forming a stable thioether bond. The compound can be used to label proteins for the detection of conformational changes, assembly of multisubunit complexes, and ligand-binding processes.