Nat. Protoc. 9, 323–336 (2014); doi: 10.1038/nprot.2014.019; published online 16 January 2014; corrected after print 2 May 2014

In the version of this article initially published, the authors omitted the following from the acknowledgments: “We thank A. Nishiyama and D. Dietrich for providing NG2-DsRed mice”. The error has been corrected in the HTML and PDF versions of the article.