Stem Cells That Kill

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Weissman and others are finding no shortage of targets. For one thing, cancer stem cells seem to be extremely mobile, able to migrate easily from their birthplace to other parts of the body, where they can churn out more stem cells and launch new tumors. Eradicating those cells at their source might help control the spread of cancers like leukemia that flare from the blood to the bone marrow and other tissues. Blocking a stem cell's source of nutrients might be another effective strategy for drug development. Unlike normal stem cells, which tap into many different blood supplies for the oxygen and growth factors they need to survive, cancerous stem cells seem to have more addictive personalities, zeroing in on one source and siphoning off everything they need. Exploiting that dependency by finding and cutting off the source would provide another way to tackle malignancies.

It could be years before any of those approaches yields an approved treatment. But interest in the field is growing rapidly, thanks in part, paradoxically, to President George W. Bush's restrictions on embryonic-stem-cell research. Some of the federal funds that might otherwise have gone to embryonic stem cells could be finding their way into cancer-stem-cell studies. "Don't expect anything before five years," says Weissman, "but be angry if you don't see anything in 15 years." Cancer patients, mark your calendars.