The ambulance brakes to a hard stop outside the doors of the emergency room, and paramedics rush to transfer a child critically injured in an auto accident. As the physicians struggle to stop internal bleeding, an X ray reveals a surprise. The liver and spleen aren't where the textbooks say they should be. Each is on the wrong side of the body. That's one of the dramatic moments in an episode of ER, the nation's most popular television show. The bizarre plotline draws from reality. As many as 1 in 8,500 people have the normal left-right placement of their organs flip-flopped. By itself, this condition, called situs inversus, rarely poses any medical problems. Nonetheless, the oddity shines light on the issue of how a growing embryo, which starts as a simple ball of cells with no asymmetries, learns its left from its right. How does the body shift the heart toward the left side of the chest, while its aorta loops to the right? What mechanism gives three lobes to the right lung, while the left has only two, apparently offering more room for the heart? As the ER episode dramatically illustrates, such asymmetry persists farther down the body: The stomach and spleen normally fill the left side of the abdominal cavity, the liver and gall bladder the right, and the intestines run from right to left. Over the past several years, developmental biologists have begun to address the origin of left-right asymmetry. Through studies of chick, frog, and mouse embryos, they've found a handful of genes that are more active on one side or the other of the early embryo (SN: 7/26/97, p. 56). Yet scientists believe that the asymmetric expression of those genes merely reflects an earlier event in which the embryo began to distinguish left and right. That original break in symmetry is what investigators are eager to understand. Some of them have speculated that an embryo derives its first knowledge of left and right from an asymmetrically shaped molecule that lines up along the embryo's other two axes, the head-tail axis and the back-front axis. Imagine placing an F-shaped molecule on your chest. As long as it is positioned consistently in regard to the other two axes, the arms of the F will distinguish between the left and right sides of your body. Several studies of mutant mice, along with a dose of medical history, now offer a seemingly different explanation: In a key part of the embryo, the twirling of cilia, hairlike extensions on cells, may generate a one-way flow of molecules that ultimately lets the developing organism tell its left and right sides apart. It's an incredibly appealing model, savs Cliff Tabin of Harvard Medical School in Boston, who studies left-right asymmetry in chick embryos.