Nat. Struct. Mol. Biol. 23, 95–97 (2016); published online 3 February 2016; corrected after print 11 February 2016
In the version of this article initially published, the name of the first author in ref. 1 was misspelled as Oldman. The correct name is Oldham. The error has been corrected in the HTML and PDF versions of the article.