Nat. Med. 13, 1382–1387 (2007); published online 28 October 2007; corrected after print 12 November 2007.

In the version of this article initially published, Equation 2 is incorrect. The correct equation is:

Additionally, peak 1 in Figure 1a is incorrectly defined in the figure legend. Peak 1 represents the natural abundance of [2-13C]pyruvate.

The errors have been corrected in the HTML and PDF versions of the article.