5(6)-CFDA, SE (full name: 5-(and-6)-Carboxyfluorescein diacetate, succinimidyl ester, also known as CFSE) 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 non-fluorescent but becomes brightly green fluorescent once it is hydrolyzed by intracellular esterases.