Nat. Struct. Mol. Biol. 21, 955–961 (2014); published online 12 October 2014; corrected after print 18 February 2015

In the version of this article initially published, Supplementary Figure 4k showed levels of mature miRNA-124 instead of miR-124 precursor. The error has been corrected in the Supplementary Text and Figures file and in the HTML and PDF versions of the article.