a, Intermolecular backbone interactions between the A1–T6 segment of bound H3(1–15)K4me3 peptide and the PHD finger in the complex. b, Intermolecular hydrogen-bonding interactions involving the guanidinium group of R2 in the complex. c, Superposition of free (coloured green) and H3(1–15)K4me3-bound complex (coloured yellow) of the BPTF PHD finger. d, e, Positioning of the trimethylated lysine of the H3(1–15)K4me3 peptide (d) and the dimethylated lysine of the H3(1–15)K4me2 peptide (e) within a four-aromatic-amino-acid cage of the BPTF PHD finger. Two bridging water molecules link the NH of K4me2 to the carboxylate of D6, as indicated by dashed lines.