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Propidium Iodide (PI) is a cell impermeant DNA binding dye. In a population of cells, there are live cells and dead cells, and PI can be used to identify dead cells in a mixed population. In this case a healthy cell membrane will exclude the dye; the cell membrane forms an intact barrier to exclude the dye from getting into the cell. When a cell is dead, the cell membrane is compromised, and the PI dye is allowed inside the cell where it can bind to the DNA and become fluorescent, and thus identify the dead cells. This uses a log scale on your flow cytometer.
When cells are fixed, PI can be used to determine DNA content cell cycle. During fixation, the cell membrane loses its integrity and so PI may freely enter the cell to bind to nucleic acids. Because PI is not DNA-selective, remember to add RNase to your staining solution. This uses a linear scale readout on your flow cytometer.