Nat. Med. 14, 1363–1369 (2008); published online 30 November 2008; corrected after print 4 December 2008

In the version of this article initially published, the title included a misspelling—'heterotropic' should have been 'heterotopic'. Additionally, the fourth and fifth sentences of the abstract were incorrectly worded and have been corrected to state more clearly the role of Ad.Cre. These changes do not affect the scientific content of the text. The errors have been corrected in the HTML and PDF versions of the article.