Cyanine3 tetrazine is a fluorescent dye with tetrazine moiety for a conjugation reaction called TCO (trans-cyclooctene) ligation. Actually, tetrazines react not only with trans-cyclooctenes, but also with (methyl)cyclopropenes and strained non-benzo-fused cycloalkynes.
Cyanine3 is a moderately hydrophobic dye, enabling labeling in non-aqueous environment as well as in aqueous media containing some organic co-solvent like acetonitrile or DMSO. For reactions in purely aqueous media, a sulfonated version of this reagent is available.