Correction to: Nature https://doi.org/10.1038/nature09642, published online 02 January 2011.

In this Letter, the inset times shown in the DIC and GFP microscopy images in Fig. 1b and c should be in minutes (‘min’) rather than seconds (‘s’) (that is, −2 min, 6 min, 20 min, 34 min and 44 min for Fig. 1b; −4 min, 2 min, 26 min, 48 min and 54 min for Fig. 1c). The original Letter has not been corrected.