During our visit in the summer of 1994 to the Chernobyl Exclusion Zone, a region within a 30 km radius of the Chernobyl Nuclear Power Plant, we were amazed by the diversity of mammals living in the shadow of the ruined reactor only eight years after the meltdown. During our excursion through the woods, we trapped some of the local mice for examination in a makeshift laboratory. We were surprised to find that, although each mouse registered unprecedented levels of radiation in its bones and muscles, all the animals seemed physically normal, and many of the females were carrying normal-looking embryos. We found that the mice did not have any obvious chromosomal damage. We wondered whether the absence of injury could be explained by some sort of adaptive change, perhaps a more efficient DNA-repair mechanism, after many prior generations had been exposed to radiation. But when we transplanted wild mice from uncontaminated regions into cages in the Exclusion Zone and then examined their chromosomes, they were likewise unaffected by the radiation. In at least this respect, the mice seemed to have a natural “immunity” to harm from radiation.