The PromoKine Cell Proliferation Kit I (CFSE) provides a convenient and sensitive way to determine cell proliferation or cell division. The assay is based on the amine-reactive dye CFDA SE (5-(and 6)-carboxyfluorescein diacetate, succinimidyl ester; also known as CFSE) which is non-fluorescent and can diffuse passively into cells. When it enters the cells, the acetate group is removed by intracellular esterases yielding the bright green-fluorescent dye carboxyfluorescein SE. Its succinimidyl ester (SE) reactive group reacts with intracellular amines and attaches the fluorescent dye to intracellular proteins, retaining it in the cells and allowing fixation with aldehyde fixatives. CFDA SE has been used as a tracer for several cell divisions as the label is stably inherited by daughter cells through successive cell divisions or after cell fusion and is not transferred to adjacent cells in a population.