Single cell electrophoresis assay performed after 4 and 24 hours incubation on X-ray irradiated cells at 0.003; 0.012; 0.048 Gy/28 kV and 5 Gy/30 kV. The percentage of DNA strand breaks observed in the mammary epithelial cell lines (A) MCF-10A and (B) MCF-7 was analyzed according to the length and thickness of comet tail and classified into four categories from 1–4 representing increasing tail intensities. The percentage of tail DNA was assigned by summing the total number of cells containing tail DNA, multiplied by its respective category number. Total analyzed cells per slide 100. (C) Example of silver stained of a nuclei with no visible lesion (1) and a tail DNA classified as lesion type 2 (2, 3) * significant difference in comparison irradiated versus non-treated cells (Student’s t test * P ≤ 0.05; ** P ≤ 0.001). (D) Table showing the percentage of cells detected in each category of damage. † Comet tail type: 0 corresponds for no visible DNA breaks and the categories 1 to 4 represents increasing amounts of DNA breaks. DS stands for Damage Score and corresponds to the total percentage of tail DNA.