Nature 509, 101–104 (2014); doi:10.1038/nature13134

In the first sentence of the main text of this Letter, the words ‘...Brg1, Bmi1 (also known as Smarca4)...’ should have read ‘...Brg1 (also known as Smarca4), Bmi1...’. This has been corrected in the PDF and HTML versions of the article.