Used in bioconjugation for site-specific labeling of biomolecules due to its fast and selective reaction with tetrazine derivatives via inverse electron-demand Diels-Alder reaction. Commonly applied in live-cell imaging, targeted drug delivery, and pretargeted imaging techniques such as PET and fluorescence imaging. The PEG4 spacer enhances solubility and reduces steric hindrance, improving reaction kinetics and biocompatibility. The primary amine group allows for easy coupling to carboxylic acids, activated esters, or other amine-reactive moieties, enabling modular construction of complex bioconjugates.