5(6)-CFDA is a cell-permeable A peractive esterase substrate that measures the enzyme activity required to activate its fluorescence and the cell membrane integrity required to retain its fluorescent product within the cell. Upon hydrolysis by intracellular esterases, this AM ester also produces carboxyfluorescein. Carboxyfluorescein contains an additional negative charge compared to fluorescein dyes and is therefore better retained in cells.