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. The dye is essentially nonfluorescent in the absence of nucleic acids, but exhibits excitation/emission maxima ~642/660 nm when bound to nucleic acids. TRhD is also used as a nuclear counterstain and dead cell indicator.

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