DAPI (4',6-diamidino-2-phenylindole) is a fluorescent dye which can bind DNA strands robustly, the fluorescence being detected by fluorescence microscope. DAPI can dye both live and fixed cells as it can cross intact membrane, with higher efficiency in fixed cells. The molecular formula is C16H15N5·2HCl with 350.25 g/mol molecular weight, CAS Number 28718-90-3. DAPI could pass through the cell and nucleic membranes and bind the double-strand DNA in the nucleus, producing 20 times stronger fluorescence than itself. The efficiency detected by fluorescence microscope is very high (almost 100%), having no side effects for the live cells. The sensitivity for double stranded DNA DAPI staining is many times larger comparing to ethidium bromide (EB) . DAPI staining is usually used in cell death detection, as it enters more effectively and generates stronger fluorescence in dead cells. After staining with DAPI, detect with fluorescence microscope or flow cytometry. Blue fluorescent cell would be seen under the microscope after staining. The largest excitation wavelength for DAPI is 340nm (ultraviolet), and the largest emission wavelength is 488nm (blue) . When DAPI binds with double-strand DNA, the largest excitation wavelength is 360nm, while the largest emission wavelength becomes 460nm. DAPI's blue emission makes it suitable for combined assays where the fluorescence ranges of DAPI and other IHC-employed fluorescent molecules like green-fluorescent fluorescein and GFP, or red-fluorescent stains like Texas Red, are completely distinctive.