Nature 515, 523–527 (2014); doi:10.1038/nature13953

In this Article, two errors were introduced into the figures during the production process. In Fig. 3i, the x axis should alternate ‘WT, 2, WT, 2’ rather than ‘WT, 6,8, WT, 6,8’, and in Fig. 2g, the 6,8 bar should have one asterisk, rather than two. These errors have been corrected in the online versions of the paper.