The amine-reactive Oregon Green™ 488-X, succinimidyl ester can be used to can be used to create green fluorescent bioconjugates with 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. This reactive dye contains an additional seven-atom aminohexanoyl spacer ('X') between the fluorophore and the succinimidyl ester group. This spacer helps to separate the fluorophore from its point of attachment, potentially reducing the interaction of the fluorophore with the biomolecule to which it is conjugated.