Acridine Orange (AO) selectively stains the acidic compartment of the cell, which was useful for nucleic acid and acidic body staining and cell cycle determination. AO interacts with DNA and RNA by intercalation or electrostatic attraction respectively. DNA intercalated AO fluoresces green (525nm); RNA electrostatically bound AO fluoresces red (>630nm). It distinguishes between quiescent and proliferating cells, and also allows differential detection of multiple G1 compartments. AO is also useful as a method for measuring apoptosis, and for detecting intracellular pH gradients and the measurement of proton-pump activity.