Edited by Thane Peterson

Prominent "port-wine stains" and other birthmarks blemish many children -- and such famous people as Cicero and Mikhail Gorbachev. Scientists have long known that these marks are caused by the aberrant growth of blood vessels under the skin. But now, a team of researchers led by cell biologist Bjorn Olsen of Harvard Medical School has figured out what goes wrong.

The culprit: a faulty gene. In the December 28, 1996, issue of the scientific journal Cell, Olsen describes how scientists came across a large family that has many members who have port-wine stains and other birthmarks. What these people had in common was a flaw in a gene dubbed TIE-2, which codes for a receptor on cells that line the insides of blood vessels.

How does the gene lead to birthmarks? The receptor appears to play a key role in the complicated process in which veins grow and branch to supply the body with blood. When flawed, this process goes awry. Growing veins never link up properly with the cells that strengthen the vein walls. The result: Incomplete veins meander wildly under the skin. In severe cases, they can form huge bulges of abnormal tissue. More commonly, however, their presence under the skin causes the characteristic port-wine mark.

The discovery sheds new light on the formation of blood vessels during development. And eventually, it may lead to new treatments that attack the root cause of birthmarks, not just the symptoms themselves. Scientists know now that "there is nothing impossible about it," says Olsen. "We can do it."

By John Carey in Washington