Nat. Biotechnol. 23, 975–982 (2005); published online 17 July 2005; corrected after print 10 October 2006

In the version of this article initially published, 'n' was incorrectly defined on p. 981, the first line after equation 3, as the “apparent number of osmotically active moles per cell.” The correct phrase is, “apparent moles of osmotically active particles per cell.” The error has been corrected in the PDF version of the article.