FITC (fluorescein isothiocyanate) is a fluorochrome dye that absorbs ultraviolet or blue light causing molecules to become excited and emit a visible yellow-green light. This emission ceases upon removal of the light causing the excitation. Fluorochrome labeling provides rapid, accurate localization of antigen-antibody interaction when one of the reactants is part of a cell, tissue or other biological structure. FITC is a commonly used marker for antibodies in immunofluorescent techniques since the conjugation of FITC to proteins is relatively easy and does not, in general, destroy the biological activity of the labeled protein. FITC is widely used as a hapten to label different proteins.