Nat. Med. 12, 917–924 (2006); published online 16 July 2006; corrected after print 7 January 2010

In the version of this article initially published, the source of the Nestin-Cre mice was incorrectly stated as Jackson Labs. The correct source of the Nestin-Cre mice (which were on a mixed 129/Sv × C57BL/6 hybrid background) was R. Klein (Max Planck Institute of Neurobiology). The error has been corrected in the HTML and PDF versions of the article.