Mal-PEG1-acid is widely used in bioconjugation and drug delivery applications. Its maleimide group allows for selective and efficient conjugation with thiol-containing molecules, such as cysteine residues in proteins or peptides. This makes it valuable for creating stable, targeted drug conjugates or modifying biomolecules for therapeutic or diagnostic purposes. The PEG spacer enhances solubility and reduces immunogenicity, improving the pharmacokinetic properties of the resulting conjugates. It is also employed in the development of antibody-drug conjugates (ADCs), where it links cytotoxic drugs to antibodies for targeted cancer therapy. Additionally, it finds use in surface modification of nanoparticles, enabling precise functionalization for biomedical applications.