Nature Communications 4: Article number: 2637 (2013); Published: 6 November 2013; Updated: 4 September 2014.

The images in Supplementary Figs S1a and S3a, that accompany this Article were incorrectly rendered while the Supplementary Information file was being prepared for publication. Supplementary Figs S1 and S3 as they should have been published appear below.