Errors have been found in Fig. 2, panel C, and in the text of the legend for Fig. 2, panels A and C. The correct versions of Fig. 2 and its legend can be found in the corrected online version of this article at http://stm.sciencemag.org/content/3/88/88ra54.full. The corrected PDF version of the article is at http://stm.sciencemag.org/content/3/88/88ra54.full.pdf.
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