Nat. Biotechnol. 33, 1159–1161 (2015); published online 5 October 2015; corrected after print 4 February 2016

In the version of this article initially published, when discussing the data in Figure 2b, on p. 1160, we wrote, “...targeting the same HBG1/2 promoter and found they had 32 and 55 perturbed transcripts....” This should have been “31 and 55 perturbed transcripts” as in the sentence in the figure legend discussing the same data. The error has been corrected in the HTML and PDF versions of the article.