Deparaffinize tissue sections in Histolemon (2x10 min). 2. Hydrate sections through graded alcohol series (100%, 95%, 70%, H2O) (1x5 min). 3. Incubate slides in EDTA at 95°C for 50 min for antigen unmasking. 4. Cool slides for 20 min at room temperature, wash in H2O. 5. Incubate slides in 3% H2O2 for 5 min at room temperature. 6. Wash twice in TBS. 7. Preincubate slides with Ab mix for 20 min at room temperature. 8. Incubate slides overnight at 4°C with anti-γH2AX in Ab mix, wash twice in TBS. 9. Incubate with secondary antibody for 30 min at room temperature, wash twice in TBS. If washing is incomplete, repeat the washing steps. 10. Incubate in peroxidase substrate solution for 5 min at room temperature. 11. Block reaction in H2O, counterstain with Hematoxylin for 15 sec, wash in running H2O. 12. Dehydrate sections through graded alcohol series, mount with Eukitt. 13. Include positive (irradiated skin) and negative (non-irradiated skin) controls in each experiment. If control results are inconsistent, repeat the staining and control steps.