Propidium iodide (PI) is a red-fluorescent nuclear and chromosome counterstain. It is not permeant to live cells, making it useful for detecting dead cells. PI binds to DNA by intercalating between the bases, with little or no sequence preference. In an aqueous solution, PI has a fluorescent excitation maximum at 493 nm (blue-green) and an emission maximum at 636 nm (red). It is commonly used in flow cytometry for cell viability and DNA content analysis, as well as in microscopy to visualize the nucleus and other DNA-containing organelles.
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