Nature 473, 221–225 (2011)

In this Letter, the scale bars were calculated incorrectly for each image throughout the manuscript and 60× objectives were reported as 63×. Although all the images are correct as published, the length represented by every scale bar was wrong. The figure legends for Fig. 1 (the three scale bars are now all 100 μm and ‘×630’ should be ‘×600’) and Fig. 2 (the scale bar is now 80 μm) have been amended in the PDF and HTML versions online. Similar errors in Supplementary Figs 2–5 and 7–10 have also been corrected online. We very much regret these errors. These changes do not alter any of the conclusions of this Letter.