What does FITC bind to?

FITC (Fluorescein isothiocyanate) labels proteins. 

FITC is a widely used fluorescence labeling reagent. It forms a covalent amide bond on reacting with a free primary amine group on the protein. The primary amine interacts with the electrophilic carbon of the isothiocyanate group, resulting in the formation of a thiourea bond, which binds the FITC to the protein.