Correction to: Nature, published online 23 May 2018.

In this Letter, we omitted a reference to an article from Benvenisty and colleagues1 that is relevant to our study. We now refer to the study in the text as follows, and the reference has been added to the reference list as ref. 7 of the original Letter, with refs 7–22 renumbered 8–23 accordingly. The sentence ‘The organizer in primates, including humans, has so far not been defined.’ has been replaced with ‘While cells exhibiting some molecular characteristics of an organizer have been generated in vitro from human embryonic stem cells7 (hESCs), these functional properties have not been demonstrated and the organizer in humans remains undefined.’ This Letter has been corrected online.