DNA fragmentation detected with agarose gel electrophoresis of DNA extracted from X-ray (6 Gy) irradiated cells. A and B represent with and without glycerol (0.6 M), respectively. C, no irradiation; Lanes 12, 24. 36 and 48, incubation periods (h) after X-ray irradiation. Lane M, øX174 DNA Hae III fragments as size markers.