The ApopTag ISOL Dual Fluorescence Kit utilizes a proprietary double hairpin, dual fluorescently labeled oligonucleotide labeling process to detect and distinguish between typical apoptotic DNA breaks induced by either DNase I or DNase II enzyme activities. The Vaccinia Topoisomerase I mediated ligation reaction is adaptable for staining paraffin-embedded tissue, frozen tissue sections, cell suspensions, and adherent cells. In situ staining for DNA fragmentation (as in the ISOL method) is both a means of detection for rare cells and an analytical test of those cells' DNA. ApopTag ISOL Kits facilitate the differentiation of apoptotic cells from necrotic or transiently damaged cells. While conventional in situ detection techniques such as ISEL (Klenow DNA polymerase), TUNEL (terminal deoxynucleotidyl transferase, TdT) and ISNT (DNA Polymerase I) are useful in detecting internucleosomal DNA cleavage, they do not differentiate DNase Type I and DNase Type II cleavage which results from the activation of apoptotic endonucleases. When ISOL was used for direct comparison with TUNEL in specimens without necrosis, the results have been concordant. In specimens presenting necrosis, the better selectivity of ISOL was proven.