5-(and-6)-Carboxy-2',7'-dichlorofluorescein diacetate, succinimidyl ester is a useful fluorescent tracer that can passively diffuse into cells and covalently label intracellular proteins, resulting in long-term cell labeling. The reagent itself is colorless and nonfluorescent but becomes brightly green fluorescent once it is hydrolyzed by intracellular esterases.