Nat. Immunol. 10, 1125–1132 (2009); published online 6 September 2009; corrected after print 5 October 2009

In the version of this article initially published, two relevant papers are not cited. The following brief description of the findings of these papers has been added to the end of the fourth full paragraph on page 1131, and the citations below are now included at the end of the reference list:

“Our observations follow the identification in human thymi and umbilical cord blood of CD3+CD4+CD161+ cells that express RORγt, IL-23R and CCR6 and produce IL-17 after activation and stimulation with IL-1 and IL-23 (ref. 54). This work suggested that thymus-derived cells could be precursors of peripheral TH-17 cells in humans, an idea supported by the finding that CD161+IL-17+ cells can be isolated from the intestinal tissue of patients with Crohn's disease55.”

The error has been corrected in the HTML and PDF versions of the article.