The retraction was published in Thursday's issue of Nature and is signed by the senior author, Amy J. Wagers, and two others. They say that serious concerns, which they did not specify, have undermined their confidence in the original report.

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Dr. Wagers issued a statement saying that she had immediately brought the disturbing information to the attention of Nature and the Harvard Medical School, and that she was working to repeat the experiments. She said by e-mail that the information came to light in the course of studies in her laboratory, prompting her to re-examine the reported data.

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Dr. Wagers has expressed her doubts about a second paper co-authored with Dr. Mayack and published in the journal Blood in August 2008. In a statement issued today, the journal said it was posting a "Notice of Concern" about the paper pending further review.