The TUNEL assay (Terminal deoxynucleotidyl transferase dUTP nick end labeling) is a laboratory technique used to detect DNA fragmentation, which is a hallmark of apoptosis (programmed cell death). This method labels the ends of broken DNA strands with fluorescent or enzymatic markers, allowing researchers to visualize and quantify cell death in tissue samples or cell cultures. During the TUNEL assay, cells are treated with a terminal deoxynucleotidyl transferase enzyme that incorporates labeled nucleotides into the free ends of the fragmented DNA. The resulting signal can be observed under a fluorescence or light microscope, providing insights into the extent of apoptosis in various biological contexts, such as disease progression or response to therapy.