Used in bioconjugation to label proteins, antibodies, or peptides for fluorescence imaging. Enables detection and tracking in cellular assays due to strong red fluorescence and high photostability. Water-soluble, allowing labeling in aqueous buffers without organic solvents. Commonly applied in flow cytometry, immunofluorescence, and live-cell imaging. The NHS ester reacts efficiently with primary amines, forming stable amide bonds. Suitable for multiplexing due to distinct excitation and emission profiles.