Nat. Biotechnol. 35, 502–506 (2017); published online 7 June 2017; corrected after print 28 February 2018

In the version of this article initially published, in the HTML, the wrong Creative Commons Attribution license (cc-by-nc-sa rather than cc-by) was inserted; no license at all appeared in the PDF. The errors have been corrected in the HTML and PDF versions of the article.