Nat. Med. 12, 446–451 (2006); published online 26 March 2006; corrected after print 7 January 2010

In the version of this article initially published, two micrographs in Figure 2c, corresponding to the conditions BSO(−) Lineage and BSO(+) Lineage, were incorrect. These micrographs have been replaced with the correct micrographs in the HTML and PDF versions of the article.