Correction to: Nature Biotechnology https://doi.org/10.1038/s41587-019-0016-3, published online 18 February 2019.

In the version of this article initially published, in Fig. 3i, right-hand side, the image of the day 14 mouse was an inadvertent duplicate of the adjacent day 7 mouse image. The day 14 image has been replaced in the HTML and PDF versions of the article.