TUNEL assay

Method
Terminal uridine deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) is a common method for detecting DNA fragmentation that results from apoptotic signaling cascades. The assay relies on the presence of nicks in the DNA which can be identified by terminal deoxynucleotidyl transferase, an enzyme that will catalyze the addition of dUTPs that are secondarily labeled with a marker. It may also label cells undergoing necrosis or cells that have suffered severe DNA damage.

History
Originally described in the paper by Garvrieli, Sherman, and Ben-Sasson, TUNEL has become one of the main methods for detecting apoptotic programmed cell death. However, for years there has been a debate about its accuracy, due to problems in the original assay which caused necrotic cells to be inappropriately labeled as apoptotic. The method has subsequently been improved dramatically and if performed correctly should only identify cells in the last phase of apoptosis.