This is a cysteine-rich domain termed ADD (ATRX-DNMT3-DNMT3L, AD-DATRX) found in DNMT3A proteins. The ADD domains of the DNMT3 family have a decisive role in blocking DNMT activity in the areas of the genome with chromatin containing methylated H3K4. Furthermore, the ADD domain of DNMMT3A (ADD-3A) competes with the chromodomain (CD) of heterochromatin protein 1 alpha (HP1alpha, CDHP1alpha) for binding to the H3 tail. The DNA methyltransferase (DNMT) 3 family members DNMT3A and DNMT3B and the DNMT3-like non-enzymatic regulatory factor DNMT3L, are involved in de-novo establishment of DNA methylation patterns in early mammalian development [1,2].

1: DNMT3L connects unmethylated lysine 4 of histone H3 to de novo methylation of DNA. Ooi SK, Qiu C, Bernstein E, Li K, Jia D, Yang Z, Erdjument-Bromage H, Tempst P, Lin SP, Allis CD, Cheng X, Bestor TH; Nature. 2007;448:714-717. PMID:17687327

2: Structural basis for recognition of H3K4 methylation status by the DNA methyltransferase 3A ATRX-DNMT3-DNMT3L domain. Otani J, Nankumo T, Arita K, Inamoto S, Ariyoshi M, Shirakawa M; EMBO Rep. 2009;10:1235-1241. PMID:19834512