Nat. Med. 22, 1033–1042 (2016); published online 22 August 2016; corrected after print 21 September 2016

In the version of this article initially published, labels for micrographs in Figure 2d and Figure 5b were omitted, and two grants were not acknowledged in the Acknowledgments section. The errors have been corrected in the HTML and PDF versions of the article.