Nature Communications 6: Article number: 10106 (2015); Published: 3 December 2015; Updated: 3 March 2016

The original version of this Article contained an error in the spelling of the author Jennifer S. Stevens, which was incorrectly given as Jennifer J. Stevens. Furthermore, in Fig. 5, the titles for panels a and b were inadvertently switched. Both of these errors have now been corrected in the PDF and HTML versions of the Article.