Used in bioconjugation for site-specific labeling of biomolecules, particularly in click chemistry applications. Its strained alkyne group reacts rapidly and selectively with tetrazines via copper-free strain-promoted inverse electron-demand Diels-Alder (IEDDA) click chemistry, making it ideal for labeling sensitive biological systems such as live cells or proteins without cytotoxic effects. The PEG4 spacer improves solubility in aqueous environments and reduces non-specific binding, enhancing performance in complex biological matrices. The NHS ester group enables efficient covalent attachment to primary amines on proteins, peptides, or amine-modified nucleic acids, allowing two-step conjugation strategies. Commonly used in the development of antibody-drug conjugates, targeted imaging probes, and functionalized nanoparticles for diagnostics and therapeutic delivery.