Nat. Struct. Mol. Biol. 15, 1130–1132 (2008); corrected after print 6 May 2009

In the version of this article initially published, the incorrect PDB code for the MthK open channel structure was provided in the legend to Figure 1. The correct PDB code for this structure is 1LNQ. The error has been corrected in the HTML and PDF versions of the article.