Cytochemical detection of DNA breakage in nuclei of conjugating Tetrahymena with fluorescence microscopy. Post-fertilization cells with 2 incipient micronuclei (mic) and 2 developing macronuclei (AN). The old macronucleus (OM) is condensed and is in the process of being eliminated. Right panel: Stained for DNA with DAPI. Left panel: Same pair of cells probed for DNA "breaks" with the ned-labeling in-situ nick translation assay, using biotinylated dUTP, and staining with fluroesceinated avidin. Incipient micronuclei are unstained. Developing macronuclei are lightly stained, as expected, testifying to DNA cleavage which occurs during normal genomic rearrangement. However, the old macronucleus is intensely stained, indicating massive digestion of DNA.