The hematopoietic system--blood and the organs that produce it--are a great place to look for the first signs of radiation damage because it's the most vulnerable. In the study, published yesterday in Science Translational Medicine, researchers exposed mice to low, medium, and lethal amounts of radiation. When they checked the rodents' blood, they found that genes floating in the blood, called microRNA, were excellent indicators for the level of radiation to which the mice had been exposed. For one kind of microRNA--there are more than 60 that have been used as biomarkers for various diseases--the code is almost identical in everyone, but radiation scrambles the code. Higher levels of radiation lead to more mutations.