text: Well characterized modifications to histones include:
  *Methylation
  Both lysine and arginine residues are known to be methylated. Methylated lysines are the best understood marks of the histone code, as specific methylated lysine match well with gene expression states. Methylation of lysines H3K4 and H3K36 is correlated with transcriptional activation while demethylation of H3K4 is correlated with silencing of the genomic region. Methylation of lysines H3K9 and H3K27 is correlated with transcriptional repression. Particularly, H3K9me3 is highly correlated with constitutive heterochromatin.
  *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
  However, there are many more histone modifications, and sensitive mass spectrometry approaches have recently greatly expanded the catalog.