Acridine Orange is a cell-permeant nucleic acid binding dye that emits green fluorescence when bound to double-stranded DNA and red fluorescence when bound to single-stranded DNA or RNA. This unique characteristic makes acridine orange useful for cell-cycle studies. Acridine orange has been widely accepted and used in many different areas, such as epifluorescence microscopy, the assessment of sperm chromatin quality. Acridine orange stain is particularly useful in the rapid screening of normally sterile specimens, and its recommended for the use of fluorescent microscopic detection of microorganisms in direct smears prepared from clinical and nonclinical materials. The staining has to be performed at an acid pH in order to obtain this differential staining effect with bacteria showing orange stain and tissue components yellow to green. Acridine orange can be used in conjunction with ethidium bromide to differentiate between viable, apoptotic and necrotic cells. Additionally, acridine orange may be used on blood samples to cause bacterial DNA to fluoresce, aiding in the clinical diagnosis of bacterial infection once serum and debris have been filtered. Acridine orange can be used in photodynamic Cancer Therapy.