The in situ apoptosis assay is a laboratory method used to detect and quantify apoptotic cells within a tissue sample or cell culture. This assay typically involves the identification of DNA fragmentation, a hallmark of apoptosis. One of the most common methods used for in situ detection of apoptosis is the TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, where the enzyme terminal deoxynucleotidyl transferase labels the 3′-hydroxyl ends of DNA breaks, commonly associated with apoptotic cells. The labeled DNA fragments can then be visualized using various detection methods like fluorescence or colorimetric staining.