Fluorescein isothiocyanate

Fluorescein isothiocyanate (FITC) is a derivative of fluorescein used in wide-ranging applications including flow cytometry. FITC is the original fluorescein molecule functionalized with an isothiocyanate reactive group (-N=C=S), replacing a hydrogen atom on the bottom ring of the structure. This derivative is reactive towards nucleophiles including amine and sulfhydryl groups on proteins.
 * [[Image:FITC-2D-skeletal.png|200px|Fluorescein isothiocyanate]]

A succinimidyl-ester functional group attached to the fluorescein core, creating NHS-fluorescein, forms another common amine reactive derivative that has much greater specificity toward primary amines in the presence of other nucleophiles. Newer derivatives of fluorescein such as Alexa 488 and DyLight 488, have been tailored for various chemical and biological applications where greater photostability, higher fluorescence intensity, or different attachment groups are needed.