TO-PRO-3 iodide (HY-117070) is a membrane-impermeant nucleic acid dye whose staining mechanism is based on specific binding to nucleic acids. While the dye exhibits a very weak fluorescence signal in its free state, its fluorescence intensity is significantly enhanced upon binding to double-stranded DNA, with maximum excitation and emission wavelengths at 642 nm and 664 nm, respectively. Because it cannot penetrate intact cell membranes, TO-PRO-3 iodide selectively labels cells with compromised membrane integrity and is therefore often used as a dead cell indicator. The dye's fluorescent properties make it particularly suitable for detection in the far-red channel of multicolor fluorescence analysis.

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