The ApoAlert DNA Fragmentation Assay Kit detects nuclear DNA fragmentation, a hallmark of apoptosis. The kit can be used with tissue sections as well as cultured cells, and results can be visualized directly by fluorescence microscopy or quantified by flow cytometry. During apoptosis, cellular endonucleases cleave nuclear DNA between nucleosomes, producing a mix of DNA fragments that vary in length by multiples of 180 to 200 bp. The ApoAlert DNA fragmentation assay incorporates fluorescein-dUTP at the free 3"-hydroxyl ends of the fragmented DNA using the TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeling) method. The fluorescein-labeled DNA can then be quantified using either fluorescence microscopy or flow cytometry.