Increased sensitivity of thymocytes of APEX2-null mice to x-ray irradiation. (A) Mice were exposed to various x-ray doses. Thymus glands were collected 21 hours later, and viable thymocytes were counted. Ratios (percentages) of the numbers of thymocytes relative to those of an unirradiated control are shown with their mean values and SD. At least 3 mice were exposed to each x-ray dose. The difference between wild-type and APEX2-null mice was statistically significant (P = .0006, 2-way ANOVA). ○ indicates wild type; ▴, APEX2-null. (B) X-ray sensitivity of APEX2-null MEF lines. APEX2-null MEF lines with pIRES1hyg:mAPEX2 (○) or pIRES1hyg (▴) were plated in triplicate at a cell density of 500/10-cm dish and exposed to x-ray irradiation. After culture for 14 days, colonies were stained with crystal violet and counted. Ratios (percentages) of the numbers of colonies relative to those of an unirradiated control are shown with their mean values and SD.