Importance of timing of injury. After 500 cGy TBI and 500 cGy lower-extremity radiation, C57BL/6 mice received transplants of 25 million bone marrow cells. Cardiotoxin was injected intramuscularly at different time points before or after transplantation into the animal (days - 2, - 1, + 2, + 7, and + 21 relative to the day of transplantation). Animals were killed 4 weeks after the last injury, and their tibialis anterior muscles were analyzed for the presence of GFP-positive fibers. Data represents mean ± standard error from a total of 6 mice per time point from 2 separate experiments. ^*P < .05, compared with other time points.