Ed Yong in The Atlantic:
At a lab in Berkeley, California, there’s a mouse with no legs. Its head, torso, and tail are normal. It just lacks limbs. It didn’t lose those limbs; it just never grew them originally. And that’s because a team of researchers led by Axel Visel at the Lawrence Berkeley National Laboratory had replaced part of its DNA—a small sequence known as ZRS—the equivalent sequence from a snake. That tiny change was enough to “serpentize” the mouse, to stop it from developing any limbs.
ZRS is not a gene itself. Rather, it’s an enhancer—a stretch of DNA that controls the activity of genes. These sequences have long been thought to drive the wide variety of body shapes found in back-boned animals. By influencing when and where genes are activated, they can produce astonishing variety from the same basic toolkit, changing everything from the length of limbs to the number of toes.
“But it’s been difficult identifying concrete examples of this,” says Visel. Enhancers are hard to identify. You can’t just eyeball a stretch of DNA and work out where the enhancers are. They also tend to sit far away from the genes that they control—they’re like a sentence in a book that changes the meaning of a paragraph several chapters away.