Carboxyfluorescein diacetate (CFDA), which was originally used to measure intracellular pH but was soon adapted for use as a cell viability indicator. Upon hydrolysis by intracellular nonspecific esterases, CFDA forms carboxyfluorescein. As compared with fluorescein, carboxyfluorescein contains extra negative charges and is therefore better retained in cells CFDA is moderately permeant to most cell membranes, with uptake greater at pH 6.2 than at pH 7.41. CFDA has been used as a viability probe with a variety of cells, including bacteria, fungi (e.g. Saccharomyces cerevisiae), spermatozoa, natural killer (NK) cells and tumor cells.