Addendum to: Nature https://doi.org/10.1038/s41586-019-1331-zPublished online 26 June 2019

The authors have alerted the editors of Nature to potential problems in the manner in which the MALDI–TOF mass spectrometry data in this Letter have been generated and processed, and these will have an impact on the conclusions that can be reliably drawn. Nature is working with the authors to resolve the matter, but in the meantime, readers are cautioned against using results from this Letter.