Used in bioconjugation and labeling applications, particularly in click chemistry reactions. It serves as a versatile linker for attaching biomolecules, such as proteins or antibodies, to other functional groups or surfaces. The methyltetrazine group enables efficient and selective inverse electron-demand Diels-Alder (IEDDA) reactions with trans-cyclooctene (TCO) or other dienophiles, making it valuable in targeted drug delivery, imaging, and diagnostics. The PEG4 spacer enhances solubility and reduces steric hindrance, while the NHS ester allows for stable amide bond formation with primary amines, facilitating covalent attachment to biomolecules.