DNA damage responses in wild-type and Ptc1^+/− mice. (A) Mice were exposed to 3 Gy IR and killed when they showed symptoms of increased intracranial pressure. Number of mice evaluated in each group is as follows: wild-type P3–4, n = 8; wild-type P5–6, n = 16; Ptc1^+/− P3–4, n = 19; and Ptc1^+/− P5–6, n = 22. The brain of each mouse was removed and inspected for the presence of tumor. All mice that died had a grossly apparent brain tumor in the cerebellum. (B and C) Wild-type and Ptc1^+/− P5–6 mice were untreated or irradiated. 1 h later, the cerebella were dissected and lysed. Each lane represents the cerebellum from a separate mouse of the indicated genotype. Extracted proteins were immunoblotted for P-Ser¹⁹⁸¹-ATM and P-Ser³¹⁷-Chk1. Membranes were reblotted with ATM and Chk1 antibodies, respectively. Molecular mass markers (in kilodaltons) are indicated on left sides of immunoblots.