CFSE is able to passively diffuse into cells. Inside the cell, its acetate groups are cleaved by intracellular esterases, and the molecules are converted to fluorescent esters. CFSE is retained within the cell and covalently couples to intracellular molecules via its succinimidyl group. Due to this covalent coupling reaction, fluorescent CFSE can be retained within the cell for an extremely long period. Also, due to this stable linkage, once the dye has been incorporated within the cell, it is not transferred to adjacent cells. CFSE is widely used for cell proliferation assays and in vivo cell tracking.