Tue Nov 10, 6:39 AM ET

An employee of WiCell Research Institute, Inc. prepares stem cells for culture at the Wisconsin National Primate Research Center at University Wisconsin-Madison, March 2009. Human embryonic stem cells may one day be used to help people recover abilities to learn and remember that are lost after radiation treatment for brain tumors, experiments on rats suggest. (AFP/Getty Images/File/Darren Hauck)

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