C ovalent modifications of nucleosomes regulate chromatinbased processes such as DNA transcription, replication and repair. Many modifications occur on the accessible histone tails that protrude from the nucleosome core particle. These modifications include acetylation and methylation of lysine residues and are recognized by 'reader' domains that recruit various proteins. The molecular basis for how reader domains recognize histone modifications has been provided by structural studies of reader domain-histone peptide complexes 1 .Some reader domains not only bind modified histone tails, but can additionally bind to DNA. Of over 20 types of reader domains, at least four (PWWP, Tudor, chromo and bromo) are known to bind both the modified histone and DNA 2,3 . These reader domains are predicted to recognize the histone modification in the context of nucleosomal DNA, with both types of interactions contributing to the affinity of the domain for modified nucleosomes. However, such multivalent binding of a reader domain was thus far not observed directly.A widespread type of reader domain that can engage in multivalent interactions is the PWWP (Pro-Trp-Trp-Pro motif) domain. This domain was identified in the protein product of the Wolf-Hirschhorn syndrome candidate gene 1 (WHSC1) and in proteins related to hepatoma-derived growth factor (HDGF) 4,5 . It was later found in the DNA methyltransferase DNMT3B and the DNA repair protein MSH6 (refs. 6,7 ). The PWWP domain is a reader of methylated histone tails and comprises a five-stranded N-terminal β-barrel and two C-terminal α-helices 8 . The β-barrel harbors a conserved aromatic cage that binds a methyl-lysine residue 9 , as also observed for other reader domains of the 'Royal' superfamily such as Tudor, chromo, Agenet and MBT domains 10 .Most PWWP domains bind the histone H3 N-terminal tail that is di-or tri-methylated at lysine 36 (H3K36me2 or H3K36me3) [11][12][13][14][15][16] . The H3K36me3 modification occurs in gene bodies of transcribed chromatin in eukaryotic species from yeast to human 17 . This modification is mainly generated by SETD2 (KMT3A), a lysine methyltransferase associated with elongating RNA polymerase II (Pol II) 18,19 . H3K36me3 has functions in transcription elongation, alternative splicing, DNA methylation, DNA damage signaling, and repression of cryptic transcription and histone exchange 20 .The PWWP domain binds methylated H3K36 histone tail peptides with much lower affinity 11,15 than other methyl-lysine reader domains such as PHD fingers 21 . To achieve high-affinity binding, PWWP domains rely on additional interactions with DNA. Indeed, the PWWP domain was first described as a DNA-binding fold 7,22 , and the PWWP domain in Pdp1 was found to bind both a methylated histone peptide and DNA 23 . Nucleosomal DNA was shown to contribute strongly to high-affinity binding of a PWWP domain to an H3K36-methylated nucleosome 24,25 , and it was predicted that the domain may bind both DNA gyres 25 . Very recently, structures were reported of the PWWP domai...