Acridine Orange is an organic compound dye that is used to help determine at what stage a cell is in during a cell cycle. The dye is commonly used in epifluorescence microscopy and can bind nucleic acids. When it is bound to double stranded DNA, a green fluorescence signal will be emitted and a red fluorescence signal will be emitted when bound to RNA or single stranded DNA. With these difference fluorescence signals, Acridine Orange is a very useful tool to study cell cycles.