The APO‑BrdU TUNEL Assay Kit provides the materials necessary to label DNA strand breaks for the detection of apoptotic cells, as well as fixed samples of positive and negative control cells for assessing assay performance. Complete protocols for flow cytometry applications are also included, although the kit may be adapted for use with fluorescence microscopy as well. Final detection of BrdU incorporation at DNA break sites is achieved through an Alexa Fluor 488 dye–labeled anti‑BrdU antibody. The kit also includes propidium iodide to determine the total cellular DNA content.

The APO‑BrdU TUNEL Assay Kit detects the DNA fragmentation of apoptotic cells by exploiting the fact that the DNA breaks expose a large number of 3´‑hydroxyl ends. These hydroxyl groups can then serve as starting points for terminal deoxynucleotidyl transferase (TdT), which adds deoxyribonucleotides in a template-independent fashion. Addition of the deoxythymidine analog 5‑bromo‑2´‑deoxyuridine 5´‑triphosphate (BrdUTP) to the TdT reaction serves to label the break sites. Once incorporated into the DNA, BrdU can be detected by an anti‑BrdU antibody using standard immunohistochemical techniques. This method of labeling DNA breaks is referred to as Terminal Deoxynucleotide Transferase dUTP Nick End Labeling, or TUNEL.