The 5-isomer of fluorescein dichlorotriazine (5-DTAF) is highly reactive with proteins. Unlike other reactive fluoresceins, 5-DTAF not only reacts with amino groups, but also reacts with thiol groups and even directly reacts with hydroxy groups such as polysaccharides and other alcohols in aqueous solution at pH above 9. Due to its high reactivity, 5-DTAF is also used to label carbohydrates.