Apoptosis is essential in many physiological processes.Several methods have been described to identify apoptotic cells.Endonucleolysis is considered as the key biochemical event of apoptosis,resulting in double-stranded,low molecular weight DNA fragments as well as single strand breaks in high molecular weight DNA.Those DNA strand breaks can be identified by labeling free 3’-OH termini with modified nucleotides in an enzymatic reaction.Terminal deoxynucleotidyl transferase(TdT)can catalyze DIG-dUTP to free 3’-OH DNA ends.The DIG-dUTP is first react with biotin-conjugated anti-digoxin antibody,which subsequently associate with streptavidin-FITC(SABC-FITC).FITC is excited at around 490-495nmand emitted visible yellow-green light at 520-530 nm.