This is a cysteine-rich domain termed ADD (ATRX-DNMT3-DNMT3L, AD-DATRX) found in ATRX proteins. Chromatin-associated human protein ATRX was originally identified because mutations in the ATRX gene cause a severe form of syndromal X-linked mental retardation called ATR-X syndrome. Mutations or knockdown of ATRX expression cause diverse effects, including altered patterns of DNA methylation, a telomere-dysfunction phenotype, aberrant chromosome segregation, premature sister chromatid separation and changes in gene expression. ATRX localizes predominantly to large, tandemly repeated regions (such as telomeres, centromeres and ribosomal DNA) associated with heterochromatin, and studies show that it directs H3.3 deposition to pericentric and telomeric heterochromatin. The ADD domain of ATRX, in which most syndrome-causing mutations occur, engages the N-terminal tail of histone H3 through two rigidly oriented binding pockets, one for unmodified Lys4 and the other for di- or trimethylated Lys9. Mutations in the ATRX ADD domain cause mislocalization of ATRX protein to heterochromatin, and this may contribute to understanding the underlying etiology of ATRX syndrome. Structure analysis of the ADD domain of ATRX revealed that it contains a PHD zinc-finger domain packed against a GATA-like zinc finger. Same structure is also found in the DNMT3 DNA methyltransferases and DNMT3L [1, 2, 3].

1: Structural consequences of disease-causing mutations in the ATRX-DNMT3-DNMT3L (ADD) domain of the chromatin-associated protein ATRX. Argentaro A, Yang JC, Chapman L, Kowalczyk MS, Gibbons RJ, Higgs DR, Neuhaus D, Rhodes D; Proc Natl Acad Sci U S A. 2007;104:11939-11944. PMID:17609377

2: Combinatorial readout of histone H3 modifications specifies localization of ATRX to heterochromatin. Eustermann S, Yang JC, Law MJ, Amos R, Chapman LM, Jelinska C, Garrick D, Clynes D, Gibbons RJ, Rhodes D, Higgs DR, Neuhaus D; Nat Struct Mol Biol. 2011;18:777-782. PMID:21666677

3: ATRX ADD domain links an atypical histone methylation recognition mechanism to human mental-retardation syndrome. Iwase S, Xiang B, Ghosh S, Ren T, Lewis PW, Cochrane JC, Allis CD, Picketts DJ, Patel DJ, Li H, Shi Y; Nat Struct Mol Biol. 2011;18:769-776. PMID:21666679