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This license does not permit commercial exploitation or the creation of derivative works without speciﬁc permission. The EMBO Journal is published by Nature Publishing Group on behalf of European Molecular Biology Organization. This article is licensed under a Creative Commons Attribution-Noncommercial- No Derivative Works 3.0 Licence. [http://creativecommons. org/licenses/by-nc-nd/3.0] 2322 The EMBO Journal VOL 28 | NO 16 | 2009 & 2009 European Molecular Biology Organization