In 1999 Jan M. Van Deursen and his colleagues at the Mayo Clinic in Rochester, Minn., wanted to see whether mangled chromosomes cause cancer. So they engineered mice deficient in a protein that helps to maintain chromosomal integrity. The rodents' coils of DNA were duly deranged. Surprisingly, though, the animals were not particularly tumor-prone. Instead they developed a strange grab bag of ills, including cataracts, dwindling muscles, rapid thinning of fat under the skin and progressive spinal curvature, that made them look like one-humped camels. They also tended to die young.