Nat. Biotechnol. 33, 31–40 (2015); published online 9 January 2015; corrected after print 14 January 2015

In the version of this article initially published, the caption for Figure 5 read “Pope Innocent VIII, likely the first patient to undergo parabiosis.” In fact, he did not undergo parabiosis, but a blood transfusion. The caption should have read “Pope Innocent VIII died in a rejuvenation attempt in 1492.” The errors have been corrected in the HTML and PDF versions of the article.