In this Letter, Dominic Grün and Sagar should have been listed as authors, with the affiliation Max Planck Institute of Immunobiology and Epigenetics (MPI-IE), Freiburg, Germany. These authors established the sequencing platform, supervised the sequencing and helped with bioinformatics (see ‘Author contributions’). This study was supported by the DFG (grant GR4980 to S.), the Behrens-Weise-Foundation (to S.), and the Max Planck Society (to S. and D.G.) (see ‘Acknowledgements’). The Letter has been corrected online.