Nat. Struct. Mol. Biol. 22, 809–814 (2015); published online 21 September 2015; corrected after print 16 October 2015

In the version of this article initially published, the support to Jonathan R. Beckwith from the US National Institutes of Health (grant GMO41883) was not present. The error has been corrected in the HTML and PDF versions of the article.