This single 5-isomer preparation of our proprietary, amine-reactive dye, Oregon Green 488 carboxylic acid, succinimidyl ester and its conjugates have green fluorescence similar to that of fluorescein but are more photostable. Additionally, Oregon Green 488 dye has a lower pK_a than fluorescein (pK_a = 4.7 versus 6.4 for fluorescein), making this bright dye less pH sensitive in the physiological range.