Exposure to radiation can wreak indiscriminate havoc on cells, tissues, and organs. Curiously, however, some tissues are more vulnerable to radiation damage than others. A new study now finds that cellular survival after radiation exposure depends on behavior of the tumor-suppressor protein p53 over time. In vulnerable tissues, p53 levels go up and remain high, leading to cell death. In tissues that tend to survive radiation damage, p53 levels oscillate up and down.