Nature 500, 468–471 (2013); doi:10.1038/nature12313

In this Letter, during the preparation of Figs 1b and 3b, and Supplementary Figs 3c and 10a, the digital western blot scans were processed to remove irrelevant lanes from single-blot images. Although all excisions/mergers originated from the same gel, these figure constructions should have been explicitly pointed out. We now include the unprocessed scans of the corresponding blots as Supplementary Information to this Corrigendum. In addition, the original GRO-seq data have been uploaded at ArrayExpress with the accession number E-MTAB-5490. The original Letter has not been corrected.