Nat. Struct. Mol. Biol. 22, 1016–1022 (2015); published online 16 November 2015; corrected after print 8 December 2015

In the version of this article initially published, there were errors in two figures. The schematic in Figure 4a was mistakenly drawn with a β1→3 linkage between the −1 GlcUA and the +1 GlcNS(6S), rather than the correct β1→4 linkage. Supplementary Figure 1 incorrectly gave the names of the GlcNX monomers as N-acetyl-α-D-glucuronic acid and N-sulfo-α-D-glucuronic acid, rather than N-acetyl-α-D-glucosamine and N-sulfo-α-D-glucosamine. These errors have been corrected in the HTML and PDF versions of the article.