DCD: In the double chromodomain (DCD), both chromodomains share their general secondary structure elements with those previously seen in HP1 and Polycomb chromodomains. The linker segment forms a novel helix–turn–helix structure that juxtaposes the two chromodomains to form a continuous surface. A total of 350 Å is buried at the interface of these tandem chromodomains. (1)

Reference
1. John F. Flanagan, Li-Zhi Mi, Maksymilian Chruszcz, Marcin Cymborowski, Katrina L. Clines, Youngchang Kim, Wladek Minor, Fraydoon Rastinejad & Sepideh Khorasanizadeh.Double chromodomains cooperate to recognize the methylated histone H3 tail. Nature.2005;438:1181–1185. PMID: 16372014.

    PHD: Distinct from the Royal family is the PHD finger, a well-characterized reader of H3K4me3. It contains a C4HC3 motif that coordinates two zinc ions in a cross-brace manner. Such an arrangement produces a globular domain with a small β-sheet and an α-helix. PHD fingers make extensive contacts with H3K4me3, imparting a high degree of specificity. (1)

Reference
1. Musselman CA, Lalonde ME, Cote J, Kutateladze TG.Perceiving the epigenetic landscape through histone readers. Nat Struct Mol Biol.2012;19(12):1218-27. PMID: 23211769.