TCO-PEG4-NHS ester is widely used in bioconjugation and click chemistry applications. It serves as a linker for attaching biomolecules, such as proteins, peptides, or antibodies, to other molecules or surfaces. The TCO (trans-cyclooctene) group enables rapid and specific reactions with tetrazine derivatives through inverse electron-demand Diels-Alder (IEDDA) reactions, making it valuable in labeling and imaging studies. The PEG4 spacer enhances solubility and reduces steric hindrance, improving the efficiency of conjugation. This compound is particularly useful in drug delivery systems, diagnostic assays, and targeted cancer therapies, where precise and stable molecular connections are essential.