Used in bioconjugation for linking molecules in drug delivery systems, particularly in antibody-drug conjugates (ADCs). The maleimide group reacts selectively with thiol groups (cysteine residues) in proteins, enabling stable attachment. The PEG12 spacer improves solubility, reduces aggregation, and extends circulation time of conjugated compounds. The terminal carboxylic acid allows further activation and coupling to amines or hydroxyls on drugs, dyes, or targeting moieties. Commonly applied in pharmaceutical research and development of targeted therapies.