Someday the ability to measure the level of stem cells in a tumor could lead to new prognostic tests that predict when a cancer is likely to accelerate, helping doctors customize treatment.

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When implanted in mice, just a few hundred of these stem cells generated a massive tumor.

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The more stem cells present, the more virulently the tumor grows:They account for 1 in 4 cells in a glioblastoma tumor, the deadliest type of brain cancer, but only 1 in 500 cells in slower-growing forms of brain cancer, Dirks found.

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One challenge with embryonic stem cells, however, is preventing them from creating tumor cells.

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