Nature Biotechnology 25, 238–243 (2007); published online 21 January 2007; corrected after print 8 August 2007

In the version of this article initially published online 21 January 2007, the y-axes of Figures 1e, 2c and 2d are labeled incorrectly. They should be labeled “% CD45.1 in PB.” The error has been corrected for all versions of the article.