Fluorescein isothiocyanate (FITC) is widely used to attach a fluorescent label to proteins via the amine group. The isothiocyanate group reacts with amino terminal and primary amines in proteins. It has been used for the labeling of proteins including antibodies and lectins. Isomer I (Fluorescein 5-isothiocyanate) has the thiocyanate group on the 4 carbon of the benzene ring, whereas isomer II (Fluorescein 6-isothiocyanate) has the thiocyanate on the 5 carbon. The two isomers are indistinguishable spectrally, either by wavelength or intensity. Isomer I is more easily isolated in pure form, so is less expensive. This may explain why isomer I is more commonly used for labeling. For many purposes, however, the mixed isomers of FITC will be perfectly suitable.