Nature 429, 771–776 (2004).

It has been drawn to our attention by Vincent Keng that the image in the bottom-left frame of Fig. 1c of this Letter presents identical data to the one above it on the right. A mistake made by the authors during compilation of Fig.1 caused the wrong bottom-left image to be used instead of the correct image, which is shown below. The results presented in this replacement micrograph do not alter the conclusions of our study.

figure 1

Figure 1