The Acridine Orange (AO) / Propidium Iodide (PI) assay is used for cell counting and viability. AO is a nucleic acid-binding fluorophore that is cell membrane permeable and suitable for selective staining of nucleated cells. PI is a nucleic acid-binding dye that is impermeable to live cells and suitable for staining dead, nucleated cells. All live, nucleated cells fluoresce green due to AO, while dead, nucleated cells are stained with both AO and PI and fluoresce red.
