Sulfo-Cyanine3 Maleimide is an orange fluorescent dye containing a maleimide group that reacts specifically with thiol (-SH) groups on proteins, antibodies, and other biomolecules, forming stable thioether bonds in neutral buffers. The sulfonate modification imparts excellent water solubility, allowing labeling of solvent-sensitive or unstable proteins without organic co-solvents. This dye is widely used for fluorescence imaging, flow cytometry, and functional protein labeling studies.