Figure 4.

atm/p21 double-null tumors exhibit increased apoptosis. (a and b) Hematoxylin and eosin staining of tumor section. ×40 magnification. (a) An atm thymic lymphoma shows uniform T lymphoblastic cells with no detectable signs of apoptosis. (b) An atm/p21 double-null thymic lymphoma shows extensive apoptosis. (c) DNA damage-induced defective G₂/M checkpoint control in atm/p21 double-null thymomas leads to increased apoptosis in thymic tumor cells. Tumor cell lines were irradiated at 0 and 20 Gy. Cells were collected at 24 hr, fixed and stained with propidium iodide, and analyzed for DNA content and apoptotic cells. G₁ and G₂ peaks are indicated as well as percentage of apoptotic cells. Similar observations were seen at 5 and 10 Gy of irradiation and from independent cell lines (not shown).