5-CFDA, AM is a cell-permeable molecule that can be used as a viability probe It is an 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. The electronically neutral AM ester of CFDA can be loaded into cells at lower concentrations than CFDA. 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.