As shown in Fig. 5B, the phosphorylation of Chk2, Atm and H2AX shortly after ionizing radiation was normal in representative lines from wt mice (lanes 1,2,7–10) but, as expected, Chk2 phosphorylation was absent in the fibrosarcomas from Atm-null mice (lanes 3–6). Here, H2AX is linked to fibrosarcoma.