Nature Reviews Drug Discovery (2018); 10.1038/nrd.2018.14

In the version of this article that was originally published online, an older version of the data set categorizing proteins into target development levels was used to create Figure 1 than the version used to create Table 1, and data from Figure 1 were referred to at several points in the text of the article. Figure 1 and the associated text have been updated to match Table 1 in the online and print versions of the article. The authors apologize for any inconvenience that this error may have caused.