Aside from the brief flashes of optimism, all parties involved in the research are quick to dampen unbounded enthusiasm. Thus far, the Harvard research has concentrated solely on breast cancer, a relatively "easy" type of tumor for clinical study.

Although breast cancer is of major scientific and clinical import--about 150,000 American women each year are afflicted with the disease--researchers are still in the dark about whether the same mechanism for tumor growth applies to all other cancers. Weidner's group is just now beginning to repeat the breast cancer research, by performing similar tests on prostate cancers.

And there are other hurdles. Before the federal government approves such drugs for treatment in humans, it requires an exhaustive series of tests to ensure against side effects. "We're just starting the bureaucratic process," Ingber says. "We hope to maybe get it into humans within the next year."

Folkman, for whom tumor repression would be a scientific coup of Nobel proportions, is likewise taking a long-term perspective. As for clinical usage of the drug, he says, "it's our goal, but it's not in sight yet."

Experts are quick to warn that cancer remains a leading killer of humans, and that the survival rate for metastasized tumors--cancers which have spread through the body--is still pitifully low.

Folkman likens the recent clinical success in combatting cancer to the nearly hopeless battle against pneumonia in the early years of this century, when the sometimes successful sulfa drug and later penicillin were introduced. Scientists then had little theoretical knowledge of pneumonia, but were nonetheless happy to have some kind of treatment, however flawed.

Today, clinicians are equally baffled by the mechanism by which an innocuous-looking tumor balloons up into a deadly cancer within the body. "Whether capillaries will grow or not grow toward a tumor may depend on one or more events that are not clearly understood at this time," the Harvard researchers wrote in the New England Journal paper.

But despite the obstacles that await them, the researchers remain optimistic about their work. As they conclude in their paper, "This information may prove useful even before we understand why it is true."