CPM (full name: 7-diethylamino-3-(4'-maleimidylphenyl)-4-methylcoumarin) is probably the most widely used blue fluorescent thiol-reactive dye. This maleimide derivative of coumarin is essentially nonfluorescent until it reacts with thiols, making it possible to quantify thiols without a separation step. CPM is a good energy acceptor from tryptophan and a good energy donor to fluorescein.