Mal-PEG5-acid is widely used in bioconjugation and chemical biology for modifying proteins, peptides, and other biomolecules. Its maleimide group enables selective reaction with thiol groups, making it valuable for creating stable thioether bonds in targeted drug delivery systems, antibody-drug conjugates, and protein labeling. The PEG spacer enhances solubility and reduces immunogenicity, while the carboxylic acid group allows further functionalization or attachment to surfaces. It is also employed in the development of biophysical probes and in the study of protein-protein interactions.