Thiazole red homodimer (TRhD), also know as TOTO-3, is a cell-impermeant, high-affinity nucleic acid stain based upon a symmetric cyanine dye dimer that is essentially nonfluorescent in the absence of nucleic acids that exhibits excitation/emission maxima ~642/660 nm when bound to nucleic acids. TRhD is also used as a nuclear counterstain and dead cell indicator.