Chromosome DNA breakage is an important marker event in cell apoptosis. A series of DNA 3'-OH terminus is produced by DNA double-strand breaks in apoptotic cells or whenever there is a gap in one strand. The fluorescen labeled dUTP can be linked to the 3' -OH terminus of the broken DNA under the action of terminal deoxyribonucleotide transferase (TdT). Fluorescence of dUTP conjugate can be detected by flow cytometer or fluorescence microscope.