Nat. Biotechnol. 33, 31–40 (2015); published online 9 January 2015; corrected after print 30 April 2015

In the version of this article initially published, Retrotope Inc. was included in Table 1 in the “Walking dead” category. Nature Biotechnology has since been informed by Retrotope management that the company raised $10 million in seed funding in 2014. Retrotope has been removed from Table 1. The error has been corrected in the HTML and PDF versions of the article.