Oregon Green® 488 carboxylic acid diacetate (carboxy-DFFDA) 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 yields Oregon Green® 488 (excitation/emission maxima ~496/524 nm). This fluorinated analog of fluorescein overcomes some of the key limitations of fluorescein, including greater photostability and a lower pK_a (pK_a ~ 4.7 versus 6.4 for fluorescein), making its fluorescence essentially pH insensitive in the physiological pH range.