The shoulder in the bottom photo belongs to a knockout mouse, so named only partly because of its brawny good looks. The one on top is from a normal mouse. Molecular biologists Se-Jin Lee and Alexandra McPherron at Johns Hopkins University School of Medicine knocked out a gene from the big mouse’s DNA. Mice without the gene doubled, and in some cases tripled, their muscle mass. Lee and McPherron discovered the gene about five years ago while examining genes known to regulate embryonic growth and development. To see exactly what the gene did, McPherron bred a line of mice that lacked it. We didn’t really know what would happen, she says. We thought maybe they would have no muscle at all. But then we saw the mutants had this funny body shape. Their shoulders and hips were very bulky. Underneath the skin, McPherron found rippling muscles. The muscle fibers were larger and more numerous than in normal mice, and even after a year the mutant mice show no other abnormalities. McPherron and Lee think their research can lead to meatier, leaner livestock, and possibly a treatment for people afflicted with muscle-wasting diseases like muscular dystrophy.