Nat. Immunol. 8, 522–531 (2007); published online: 1 April 2007; corrected after print 30 May 2007

In the version of this article initially published, the second subheading on page 523 is incorrect. The correct subheading should be TSLP stimulates the population expansion of late pro–B cells. The error has been corrected in the HTML and PDF versions of the article.