Histone H3 has a long N-terminal domain that contains multiple sites of serine and threonine phosphorylation, arginine and lysine methylation, and lysine acetylation.  The methylation sites are particularly important in epigenetic programming.  Lysine 9 methylation is associated with the binding of the HP1 family of heterochromatin proteins.  Lysine 27 methylation is associated with polycomb group regulation and is important in the maintenance of the pluripotent epigenetic state.  Lysine 4 methylation functions in opposition to lysine 27 methylation and is associated with gene activation during differentiation.