Gene tells left from right.

Consider that each side of the body has its own eye, ear, arm, breast, and leg. Beneath the skin, however, this remarkable symmetry largely vanishes. The heart occupies the left side of the chest; the liver resides on the right. The right lung has fewer lobes than the left.

Biologists trying to explain how left-right asymmetries arise have recently discovered several genes that seem to prefer to act in just one side of a developing embryo (SN: 9/30/95, p. 223). In the mouse, for example, a gene active in the left portion of the growing embryo has earned the name lefty.

Whether such genes govern the establishment of left-right asymmetry or are merely turned on by other genes that actually control the process has been difficult to answer. Now, Hiroshi Hamada of Osaka University in Japan and his colleagues, the group that discovered lefty several years ago, have made mice that lack the gene. In these mutant mice, the internal organs and blood vessels go askew in a variety of usually fatal ways, reports Hamada.

"This is the first demonstration that one of these genes is absolutely required" for normal left-right asymmetry, comments Randy Johnson of M.D. Anderson Cancer Center in Houston, who has studied the phenomenon in chick embryos.

Hamada believes the protein encoded by lefty governs whether other genes are active on the embryo's left or right side. Next, Hamada and his colleagues plan to identify proteins that turn lefty on and off and thus move even closer to the origin of the developmental cascade that initiates the asymmetries.

Mutations in the first gene or genes in this cascade may help explain the rare occurrences of children born with their internal organs inverted along the left-right axis, a birth defect that generates remarkably few medical problems.