Nat. Biotechnol. 27, 387–394 (2009); published online 29 March 2009; corrected after print 8 May 2009.

In the version of the article initially published, the IC50 value of cephaeline is given as 2.7 μM in Figure 3d. The correct value is 27 μM. The error has been corrected in the HTML and PDF versions of the article.