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==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 4<sup>19</sup> 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.{{cn|date=March 2023}}
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.<ref name="pmid18552846">{{cite journal |vauthors=Wang Z, Zang C, Rosenfeld JA, Schones DE, Barski A, Cuddapah S, etal | title=Combinatorial patterns of histone acetylations and methylations in the human genome. | journal=Nat Genet | year= 2008 | volume= 40 | issue= 7 | pages= 897–903 | pmid=18552846 | doi=10.1038/ng.154 | pmc=2769248 }}</ref> Therefore, patterns of histone modifications do occur but they are very intricate, and we currently have detailed biochemical understanding of the importance of a relatively small number of modifications.
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