The tetrazine-trans-cyclooctene (TCO) ligation constitutes a non-toxic biomolecule labeling method of unparalleled speed. A tetrazine-functionalized molecule reacts with a TCO-functionalized molecule, forming a stable conjugate via a dihydropyrazine moiety. This inverse electron demand cycloaddition reaction has gained popularity due to the potential for extremely fast cycloaddition kinetics with TCO as the dienophile. AAT Bioquest offers a group of tetrazine- and TCO-containing dyes for exploring various biological systems that can use this poweful click reaction. Cy5-tetrazine has been used to label biological molecules for fluorescence imaging and other fluorescence-based biochemical analysis. It is widely used for labeling peptides, proteins and oligos etc.