Nature 438, 1030–1035 (2005); doi:10.1038/nature04184

In Fig. 3p and q and Supplementary Table 3 of this Letter, the data for sqt MO2 and sqt MO3 were inadvertently mislabelled and should be swapped. The text on page 1033 that describes Fig. 3p and q should therefore read ‘sqt MO2’ not ‘sqt MO3’ on lines 4 and 13 of the second full paragraph of the left column. All our other figures and conclusions stand. In addition, Fig 1a–c and the labelling of sqt MO2 and sqt MO3 is correct in our Reply to the Brief Communication Arising (Nature 450, E2–E4 (2007); doi:10.1038/nature06315). We regret the error.