Joseph Webster and Mark Kukuruga asked about the viability of cells stained with Hoechst 33342 for DNA content and sorted after measurement in a UV illuminating beam. There was an old publication from Los Alamos which, if I remember correctly, concluded that CHO cell viability was decreased substantially at laser powers above 100 mW; this presumably involves interaction of the UV light with the DNA-bound dye. There is much older literature showing that acridine-stained cells can be killed by blue light, and that cells stained with methylene blue (which does bind to DNA) can be killed with red light. The same cells are not harmed substantially by brief exposure to the dyes if they are not also exposed to light. Since DAPI does not normally get into viable eukaryotic cells, there should not be much reason to worry about effects on the nonstaining viable cells when DAPI is used as a "nonviability" indicator. -Howard
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