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TO-TAP-3 (Thiazole Red Monomer, also known as TO-PRO^®-3) is a far-red fluorescent carbocyanine monomeric dye. TO-TAP-3 is a cell-impermeant nucleic acid stain that is nonfluorescent in the absence of nucleic acids but exhibits a significant fluorescence enhancement upon binding to dsDNA.

TO-TAP-3 has a bright fluorescence signal and low background. The dye is useful for staining nucleic acids on microarrays, as well as for nuclear and chromosome counterstaining in microscopy. It may be used for monitoring of cell viability and dead cell detection in culture. The long-wavelength fluorescence of TO-TAP-3 is well separated from green and red fluorophores, which makes it ideal for multicolor fluorescence labeling experiments.