Widely used blue fluorescent thiol-reactive dye (Ex/Em: 384/470nm). This maleimide derivative of coumarin is essentially non-fluorescent until it reacts with thiols, making it possible to quantify thiols without a separation step. Good energy acceptor from tryptophan and a good energy donor to fluorescein. Used to monitor release of thiols and to distinguish proliferating cancer cells by nucleolar protein staining.