Nature 497, 647–651 (2013); doi:10.1038/nature12139

While revising our manuscript prior to formal acceptance, we accidentally removed two references from the main text. The footnote to Table 1 of this Letter should cite ref. 1 (not ref. 24) for footnote symbol † and ref. 2 (not ref. 21) for footnote symbol ‡. We apologise for any inconvenience.