Acridine orange is an organic compound. It is used as a nucleic acid-selective fluorescent cationic dye useful for cell cycle determination. Being cell-permeable, it interacts with DNA and RNA by intercalation or electrostatic attractions respectively. When bound to DNA, it is very similar spectrally to fluorescein, with an excitation maximum at 502 nm and an emission maximum at 525 nm (green). When it associates with RNA, the excitation maximum shifts to 460 nm (blue) and the emission maximum shifts to 650 nm (red). Acridine orange will also enter acidic compartments such as lysosomes and become protonated and sequestered. In these low pH conditions, the dye will emit orange light when excited by blue light. Thus, acridine orange can be used to identify engulfed apoptotic cells, because it will fluoresce upon engulfment. The dye is often used in epifluorescence microscopy.