Competition effect after sequential carbon ion irradiation at the Munich ion mirobeam SNAKE. BJ1-hTERT cells (normal foreskin fibroblasts immortalized by ectopic expression of the catalytic subunit of the telomerase enzyme) were first irradiated in horizontal line pattern and after 45 min re-irradiated in vertical line pattern. The irradiation pattern is well reflected after immunofluorescence detection of γ-H2AX (green, quadrant II first irradiation only, quadrant III second irradiation only, quadrant I both irradiations, and quadrant IV no irradiation). In contrast, in cells irradiated twice 53BP1 foci (red) develop only at damage sites induced during the first irradiation. 53BP1 foci do not form in response to the second irradiation, although post-irradiation incubation was long enough to allow foci formation in cells that received only the second irradiation (compare quadrant I and III).