The DeadEnd Colorimetric TUNEL System is a modified TUNEL Assay designed to provide simple, accurate and rapid detection of apoptotic cells in situ at the single-cell level. The DeadEnd Colorimetric TUNEL System measures nuclear DNA fragmentation, an important biochemical indicator of apoptosis. The system can be used to assay apoptotic cell death in both tissue sections and cultured cells. The DeadEnd Colorimetric TUNEL System end-labels the fragmented DNA of apoptotic cells using a modified TUNEL (TdT-mediated dUTP Nick-End Labeling) assay. Biotinylated nucleotide is incorporated at the 3'-OH DNA ends using the enzyme Terminal Deoxynucleotidyl Transferase (TdT). Horseradish-peroxidase-labeled streptavidin (Streptavidin HRP) is then bound to these biotinylated nucleotides, which are detected using the peroxidase substrate, hydrogen peroxide, and the stable chromogen, diaminobenzidine (DAB). Using this procedure, apoptotic nuclei are stained dark brown.