Acridine Orange base is a cell-permeable fluorescent dye that stains organisms (bacteria, parasites, viruses, etc.) bright orange and, when used under appropriate conditions (pH=3.5, Ex=460 nm), distinguishes human cells in green for detection by fluorescence microscopy. Acridine Orange base fluoresces green when bound to dsDNA (Ex=488, Em=520-524) and red when bound to ssDNA (Ex=457, Em=630-644) or ssRNA (Ex=457, Em=630-644), also can be used in cell cycle assays.