of DNA double-strand breaks, and H4K20me2 facilitates repair of such breaks by non-homologous end joining. Acetylation—by HAT (histone acetyl transferase); deacetylation—by HDAC (histone deacetylase): Acetylation tends to define the 'openness' of chromatin as acetylated histones cannot pack as well together as deacetylated histones. Phosphorylation Ubiquitination SUMOylation However, there are many more histone modifications, and sensitive mass spectrometry approaches have recently greatly expanded the catalog. A very basic summary of the histone code for gene expression status is given below (histone nomenclature is described here): Histone H2B H2BK5ac Histone H3 H3K4me1 - primed enhancers H3K4me3 is enriched in transcriptionally active promoters. H3K9me2 -repression H3K9me3 is found in constitutively repressed genes. H3K27me3 is found in facultatively repressed genes. H3K36me H3K36me2 H3K36me3 is found in actively transcribed gene bodies. H3K79me2 H3K9ac is found in actively transcribed promoters. H3K14ac is found in actively transcribed promoters. H3K23ac H3K27ac distinguishes active enhancers from poised enhancers. H3K36ac H3K56ac is a proxy for de novo histone assembly. H3K122ac is enriched in poised promoters and also found in a different type of putative enhancer that lacks H3K27ac. Histone H4 H4K5ac H4K8ac H4K12ac H4K16ac H4K20me H4K91ac Complexity Unlike this simplified model, any real histone code has the potential to be massively complex; each of the four standard histones can be simultaneously modified at multiple different sites with multiple different modifications. To give an idea of this complexity, histone H3 contains nineteen lysines known to be methylated—each can be un-, mono-, di- or tri-methylated. If modifications are independent, this allows a potential 419 or 280 billion different lysine methylation patterns, far more than the maximum number of histones in a human genome (6.4 Gb / ~150 bp = ~44 million histones if they are very tightly packed). And this does not include lysine acetylation (known for H3 at nine residues), arginine methylation (known for H3 at three residues) or threonine/serine/tyrosine phosphorylation (known for H3 at eight residues), not to mention modifications of other histones. Every nucleosome in a cell can therefore have a different set of modifications, raising the question of whether common patterns of histone modifications exist. A study of about 40 histone modifications across human gene promoters found over 4000 different combinations used, over 3000 occurring at only a single promoter. However, patterns were discovered including a set of 17 histone modifications that are present together at over 3000 genes. Therefore, patterns of histone modifications do occur but they