Correction to: Nature Biotechnology https://doi.org/10.1038/s41587-021-00823-9, published online 9 February 2021.

In the version of this article initially published, the image was credited to John Flannery, UC Berkeley. The correct credit is as follows: Reprinted with permission from B. Roska & J.-A. Sahel, Nature https://doi.org/10.1038/s41586-018-0076-4 (2018), Springer Nature. The error has been corrected in the PDF and HTML versions of the article.