Terminal Transferase dUTP Nick End Labeling (TUNEL) is an in situ method for detecting areas of DNA which are nicked during apoptosis. Terminal deoxynucleotidyl transferase is used to add labelled dUTP, to the 3 prime OH ends of single- or double-stranded DNA in a template-independent manner. The terminal deoxynucleotidyl transferase nick end labelling, or TUNEL, assay labels apoptosis at the single cell level, making it more sensitive than agarose gel electrophoresis for analysis of DNA fragmentation.