Correction to: Nature Biotechnology https://doi.org/10.1038/s41587-020-0453-z, published online 16 March 2020.

In the version of this article initially published online, the edited bases at site 18 in Fig. 2e were labeled A6 and A8; they are A9 and A11, respectively. In Supplementary Fig. 6, the x-axis labels for site 18 read, from left to right, A2, A3, A4, A6, A8, A16, A17, A19 and A20; the correct labels are A5, A6, A7, A9, A11, A19, A20, A22 and A23. The errors have been corrected in the print, PDF and HTML versions of the article.