During apoptosis, DNA is fragmented following the apoptotic activation of intracellular endonucleases. The In Situ Apoptosis Detection Kit is designed to detect fragmented DNA histochemically by TUNEL (TdT-mediated dUTP Nick End Labeling). Via this method, fluorescein-labeled nucleotides are incorporated in situ onto the 3' ends of DNA fragments, allowing histologic localization and detection of individual apoptotic cells.