Maleimide-PEG4-NHS Ester is widely used in bioconjugation and protein modification due to its dual reactive groups. The NHS ester group enables efficient coupling with primary amines, such as lysine residues in proteins or peptides, while the maleimide group allows for selective conjugation with thiol groups, like those in cysteine residues. This makes it a valuable tool for creating stable and site-specific conjugates in antibody-drug conjugates (ADCs), peptide labeling, and crosslinking applications. Its PEG4 spacer enhances solubility and reduces steric hindrance, improving reaction efficiency. Additionally, it is employed in the development of diagnostic probes, targeted drug delivery systems, and biomaterial functionalization.