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Except where otherwise noted, this item's license is described as <p>Copyright 2013, Published by Cold Spring Harbor Laboratory Press. This article is distributed exclusively by Cold Spring Harbor Laboratory Press for the first six months after the full-issue publication date (see<a href="http://genome.cshlp.org/site/misc/terms.xhtml">http://genome.cshlp.org/site/misc/terms.xhtml</a>). After six months, it is available under a Creative Commons License (Attribution-NonCommercial 3.0 Unported License), as described at <a href="http://creativecommons.org/licenses/by-nc/3.0/">http://creativecommons.org/licenses/by-nc/3.0/</a>.</p>