Background: H2AX is a histone variant that is systematically found and ubiquitously distributed throughout the genome. DNA double-strand breaks (DSBs) induce phosphorylation of H2AX at serine 139 (γH2AX), an immunocytochemical assay with antibodies recognizing γH2AX has become the gold standard for the detection of DSBs. The importance of this assay to investigate different individual responses to gamma irradiation was reviewed and an example of different radiation responses of ductal carcinoma tumors with different expression levels of ATM and HER-2 was discussed. Materials and Methods: The ductal carcinoma breast tissues were exposed to 4 Gy gamma rays and after 24 hours incubation in modified RPMI 1640 medium in 37 °C with CO₂, the frequency of residual induced DSB was assessed using γH2AX assay compared to pair normal adjacent and control breast tissues. Results: Results showed that the frequency of DSB dramatically increased in both tumor and normal irradiated tissues, compared to sham non-irradiated controls. Tumors with HER-2 over expression showed significantly lower residual DSB frequencies after 24 hours post irradiation incubation time, whereas this frequency dramatically increased in ATM under expressed tissues. Conclusion: Our data showed that different tissues may have different radio-sensitivity and ATM under- and HER-2 over-expression may lead to higher and lower sensitivity to ionizing radiation, respectively. This may be due to the role of ATM in DSB repair and HER-2 in EGFR downstream signaling pathway that with the use of cell survival mechanisms ends to resistance against radiation effects and activation of PI3K/ACT that leads to DSB repair.