Correction to: Nature Biotechnology https://doi.org/10.1038/s41587-019-0193-0, published online 22 July 2019.
In the version of this article initially published online, both C’s were underlined in the key following the blue rectangle in Figs. 4b, 5a,c and 6a. Only the second C should have been underlined. The error has been corrected in the print, PDF and HTML versions of this article.