4-Acetamido-4’-isothiocyanatostilbene-2,2’-disulfonic acid, sodium salt (SITS) is a stilbene disulfonate derivative widely used in biochemical and cellular research as an irreversible inhibitor of anion transporters, such as the anion exchanger 1 (AE1 or band 3 protein) in red blood cells and kidney cells. Its isothiocyanate group reacts covalently with primary amine groups (e.g., lysine residues) on target proteins, enabling specific labeling and blocking of transport functions. The disulfonic acid groups provide high water solubility and minimize nonspecific interactions, making it ideal for aqueous biological systems. SITS exhibits fluorescence properties (excitation ~380 nm, emission ~460 nm), allowing its use in fluorescence microscopy, flow cytometry, and studies of membrane transport dynamics, including Cl^-/HCO₃^- exchange. It is also applied in immunoassays and as a precursor for synthesizing advanced fluorescent probes for real-time monitoring of cellular processes.