Fluorescein isothiocyanate (FITC) is a derivative of fluorescein used in wide-ranging applications[1][2] including flow cytometry. First described in 1942,[3] 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. It is typically available as a mixture of isomers, fluorescein 5-isothiocyanate (5-FITC) and fluorescein 6-isothiocyanate (6-FITC).