CF® dye with TCO (trans-cyclooctene) reacts with tetrazine containing molecules via copper free click chemistry. This copper free bioorthogonal reaction is useful for labeling live cells, or when there are concerns about native protein function loss with copper in cell extracts. 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®640R dye has excitation/emission at 642/662 nm.