CF® dye with tetrazine reacts with cyclooctene (TCO, methylcyclopropene) labeled molecule via copper free click chemistry. This copper free bioorthogonal reaction is useful for staining the surface of live cells or when there are concerns about native protein function loss with copper in cell extracts. Tetrazine allows faster reaction kinetics than methyltetrazine. CF® dyes are Biotium's line of next-generation fluorescent dyes with advantages in brightness, photostability, and conjugate specificity compared to other fluorescent dyes. Far-red fluorescent CF®647 dye has excitation/emission at 650/665 nm.