You can't help but wonder, how can that How can so few changes make such a big difference? researchers working with fruit bats and mice think they have an answer. And it lies less in the animals' genes than in the short snippets of DNA that control when and where and how vigorously genes are turned on. If you've ever thought that a bat is basically just a rodent with wings, you're not too far from the mark. One of the most obvious differences between bats and mice is their forearms. Mice have these stubby little legs, and bats have these large leathery wings. But even those differences are not as major as you might imagine. Mice and bats both have a gene called Prxl, which regulates limb development. But the gene is more active in the budding wings of bats. So the researchers took the piece of DNA that control Prxl activity from a bat and stuck it into a mouse. The result pictured in the January 15 issue of Genes and Development.