5(6)-CFDA is a cell-permeant esterase substrate that can serve as a viability probe that measures both enzymatic activity, which is require to activate its fluorescence, and cell-membrane integrity, which is required for intracellular retention of their fluorescent product. Upon hydrolysis by intracellular esterases, this AM ester also yields carboxyfluorescein. As compared with fluorescein, carboxyfluorescein contains extra negative charges and is therefore better retained in cells.