This is a water-soluble heterobifunctional crosslinker featuring an N-hydroxysuccinimide (NHS) ester for selective reaction with primary amines (e.g., on lysine residues of proteins) and a 2-pyridyldisulfide group for conjugation with thiols (e.g., cysteine residues). The disulfide bond enables cleavable linkages under mild reducing conditions, such as with DTT or TCEP. The sulfonic acid group improves aqueous solubility, making it ideal for physiological buffers. Commonly used in biochemistry and molecular biology for protein labeling, antibody conjugation, surface immobilization, and bioconjugate synthesis in applications like drug delivery, diagnostics, and affinity chromatography.