CF™620R maleimide is a far-red rhodamine-based fluorescent dye with a thiol-reactive group. The maleimide group permits the dye to be attached to a thiol group via a thioether bond under mild conditions. CF™620R is highly fluorescent and extremely photostable. With its absorption and emission maxima centered at 617 and 639 nm, respectively, the dye may be used as an excellent energy acceptor in fluorescence resonance energy transfer (FRET), or used in multi-color detection applications where the excitation and emission windows can be matched with the spectral profiles of the dye for maximal fluorescence collection. The exceptional water solubility of the dye facilitates bioconjugation in aqueous media and better retains the biological specificity of the conjugates.