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This study was supported by Contract/Grant No. 30700699 between the National Academy of Sciences and the Andrew W. Mellon Foundation; Contract/Grant No. 9903375 between the National Academy of Sciences and The Robertson Foundation; Contract/Grant No. 07-90086-000-GEN between the National Academy of Sciences and the John T. and Catherine D. Mac-Arthur Foundation; Contract/Grant No. 20080270 between the National Academy of Sciences and the Ewing Marion Kauffman Foundation; Contract/Grant No. 1007160 between the National Academy of Sciences and the Burroughs Wellcome Fund; Contract/Grant No. 2007129 between the National Academy of Sciences and the Doris Duke Charitable Fund; The High Q Foundation, Myelin Repair Foundation, the FasterCures Center of the Milken Institute, and an anonymous foundation. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project.

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COVER DESIGN: Athena, goddess of wisdom (symbolized by her owl), supporting Heracles (Hercules) in the course of his twelve labors, which earned him immorality. Medallion from a drinking cup, 480-470 BC, in the collection of the Staatliche Antikens ammlungen, Munich, Germany.

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