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In the original version of this article, the statement that CCCTC-binding factor (CTCF) is conserved in most bilaterians was incorrectly referenced. Reference 58 has now been corrected in the online version of the article to cite Heger, P., Marin, B., Bartkuhn, M., Schierenberg, E. & Wiehe, T. The chromatin insulator CTCF and the emergence of metazoan diversity. Proc. Natl Acad. Sci. USA 109, 17507–17512 (2012). The authors apologize for this error.