Nat. Protoc. 9, 2206–2223 (2014); doi:10.1038/nprot.2014.152; published online 21 August 2014; corrected after print 12 September 2014

In the version of this article initially published, the source from which Figures 2, 4, 8 and 9 was adapted was not cited and credited correctly. The error has been corrected in the HTML and PDF versions of the article.