Nat. Struct. Mol. Biol. 16, 550–557 (2009); published online 19 April 2009; corrected after print 18 May 2009

In the version of this article initially published, the first sentence referring to Figure 2c (page 552, first paragraph) was incorrect. It should read: “To further test the stochastic binding of nucleotides, we simultaneously added a constant amount (5 μM) of labeled ADP(+Mg2+) and increasing concentrations of unlabeled AMP-PNP(+Mg2+) to MSH2–MSH3 (Fig. 2c, left).” The error has been corrected in the HTML and PDF versions of the article.