The halo assay consists of incubatingcell nuclei with a fluorescent DNA intercalating agent like propid-ium iodide and in measuring the increase of nuclear size causedby the radiation-induced induction of SSB. Chromatin relaxes withincreasing supercoils up to the level above which chromatin windsin the opposite sense so rewinds. The assessment of the radiation-induced increases of halo diameter made possible the detectionof chromatin condensation impairment in a number of geneticsyndromes (like AT) and permitted correlation of DSB repair andchromatin response to radiation. Progressively, the halo assay wasreplaced by the comet assay that combines halo assay and gel elec-trophoresis