Used in bioconjugation to link peptides, proteins, or other biomolecules via thiol-maleimide chemistry. The maleimide group reacts selectively with thiol groups (–SH), commonly found in cysteine residues, while the NHS ester end reacts efficiently with primary amines (–NH₂), such as those in lysine residues or amine-modified molecules. The PEG4 spacer improves solubility, reduces aggregation, and increases the flexibility and stability of the resulting conjugate. Commonly applied in the development of antibody-drug conjugates (ADCs), peptide labeling, and surface immobilization in biosensors or diagnostic platforms. Also utilized in crosslinking for protein-protein interaction studies and in the preparation of functionalized nanoparticles for targeted delivery.