fluorescein isothiocyanate label fluorescein isothiocyanate labbeled FITC labelled MI:0377 PSI-MI Fluorescein isothiocyanate is a yellow-green coloured low molecular weight dye which couples to proteins via reaction with primary amine groups at high pH. FITC is excitable at 488nm, close to its absorption maximum at 494nm, and produces maximum fluorescence emission around 520nm. fitc labelled fluorescein label