Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-018-0113-x, published online 27 August 2018.
In the version of this article initially published, the label above the top right plot in Fig. 3b (HXB2-Alexa Fluor 488) was incorrect. The correct label is ‘HXB2-Alexa Fluor 405’. The error has been corrected in the HTML and PDF versions of the article.