Nat. Biotechnol. 26, 1041–1045 (2008); published online 17 August 2008; corrected after print 9 October 2008

In the version of this article initially published, a negative sign was omitted before the expression Punified(a, b) in the text on page 1043, col. 1, line 20 and the color key in Figure 2 was upside down. These errors have been corrected in the HTML and PDF versions of the article. In addition, a sentence was added to the legend of Figure 2 to clarify the significance of positive and negative values of Punified(a, b).