An international team that includes researchers from the National Human Genome Research Institute (NHGRI), part of the National Institutes of Health (NIH), has discovered that mammalian chromosomes have evolved by breaking at specific sites rather than randomly as long thought--and that many of the breakage hot spots are also involved in human cancer.?In a study published in the July 22 issue of the journal Science, a team of 25 scientists from the United States, France, and Singapore compared the organization of the chromosomes of eight mammalian species: human, mouse, rat, cow, pig, dog, cat, and horse. Using sophisticated computer software to align and compare the mammals' genetic material, or genomes, the team determined that chromosomes tend to break in the same places as species evolve, resulting in rearrangements of their DNA. Prior to the discovery of these breakage hot spots, the prevailing view among scientists was that such rearrangements occurred at random locations.