Fluorescein isothiocyanate (FITC) remains one of the most popular fluorescent labeling reagents. Oligonucleotide conjugates of FITC are frequently employed as hybridization probes. Peptide conjugates of FITC and other fluorescent isothiocyanates are susceptible to Edman degradation, making them useful for high-sensitivity amino acid sequencing, FITC-labeled amino acidd and peptides have been separated by capillary electrophoresis with a detection limit of fewer than 1000 molecules. FITC has also been used to detect proteins in gels and on nitrocellulose membranes and is a selective inhibitor of several membrane ATPases.