Nat. Immunol.; doi:10.1038/ni.2708; corrected online 23 September 2013

In the version of this article initially published online, the affiliation of author Bart Vanhaesebroeck was incorrect. The correct affiliation is as follows: Center for Cell Signaling, Barts Cancer Institute, Queen Mary University of London, London, UK. The error has been corrected for the print, PDF and HTML versions of this article.