Nat. Med. 18, 54–58 (2012); published online 6 January 2012; corrected after print 5 April 2012

In the version of this article initially published, the affiliation of one author (R.M.R.) was incorrectly listed. The correct affiliation is Lerner Research Institute and Mellen Center for Multiple Sclerosis Treatment and Research (Neurological Institute). The error has been corrected in the HTML and PDF versions of the article.