Iodoacetyl-PEG4-NHS ester is widely used in bioconjugation and protein modification due to its reactive functional groups. The iodoacetyl group specifically targets thiol groups (-SH) in cysteine residues, enabling site-specific labeling of proteins, peptides, or antibodies. This makes it valuable for creating stable thioether linkages in biochemical studies. The NHS ester moiety reacts with primary amines (-NH2) present in lysine residues or the N-terminus of proteins, allowing for dual functionality in conjugation strategies. The PEG4 spacer enhances solubility and reduces steric hindrance, improving the efficiency of the reaction. This compound is particularly useful in drug delivery systems, where it facilitates the attachment of therapeutic agents to targeting molecules. It is also employed in the development of antibody-drug conjugates (ADCs) and fluorescent or biotinylated probes for imaging and diagnostic applications. Its versatility makes it a key reagent in research areas such as proteomics, cell biology, and nanotechnology.