Nature 452, 45–50 (2008)

Errors and inappropriate manipulations were made in the assembly and processing of images in this Article. This affected Figure 2b, d, and some of the Supplementary Figures. These errors have been rectified using original data, and the correct Figure 2 is shown below. The results and conclusions of the paper are not affected by correction of the figures. We apologize for the mistakes made, which included the overuse of Photoshop contrast levels to generate a uniform white background, and errors in figure assembly.

Figure 1
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