Fluorescein isothiocyanate isomer I is widely used as a fluorescent labeling agent in biological and medical research. Its primary application is in the conjugation with antibodies, proteins, or other biomolecules to enable their visualization under fluorescence microscopy. This labeling technique is essential for studying cellular structures, tracking molecular interactions, and diagnosing diseases. It is particularly valuable in flow cytometry for cell sorting and analysis, as well as in immunofluorescence assays to detect specific antigens in tissues or cells. Additionally, it serves as a critical tool in molecular biology for DNA sequencing and hybridization studies. Its high fluorescence intensity and stability make it a preferred choice for various diagnostic and research applications.