Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) is an in situ method for detecting DNA fragmentation associated with apoptosis. The assay uses terminal deoxynucleotidyl transferase to add labelled dUTP to the 3′-OH ends of single- or double-stranded DNA in a template-independent manner, allowing apoptotic cells to be identified at the single-cell level and with greater sensitivity than agarose gel electrophoresis for the analysis of DNA fragmentation.