Oregon Green® 488 carboxylic acid diacetate, succinimidyl ester (carboxy-DFFDA, SE) is a fixable-cell-permeant, Oregon Green®-based tracer for very long-term cell labeling. This fluorinated analog of fluorescein overcomes some of the key limitations of fluorescein, including greater photostability and a lower pKa (pK_a ~ 4.7 versus 6.4 for fluorescein), making its fluorescence essentially pH insensitive in the physiological pH range.