Learn how to use this product Acridine Orange/Propidium Iodide Stain is a cell viability dye that can distinguish viable and nonviable cells. Acridine Orange/Propidium Iodide Stain causes viable nucleated cells to fluoresce green and nonviable nucleated cells to fluoresce red. It can be used to assess cell viability with the automated fluorescence cell counters of the LUNA™ family. Acridine orange (AO) permeates viable and nonviable cells and binds to nucleic acids. Binding to dsDNA causes AO to fluoresce green, and binding to ssDNA or RNA causes it to fluoresce red. Propidium iodide (PI) binds to nucleic acids. Because PI cannot permeate intact cell membranes, PI is taken up by nonviable cells and cells with compromised membranes. Once bound to nucleic acids, a 20-30-fold increase in fluorescence takes place, causing the cell to fluoresce red. Due to Förster resonance energy transfer (FRET), the PI signal absorbs the AO signal in nonviable cells, ensuring no double positive results.