Acetylation and deacetylation of lysine are very important reversible modifications controlling protein activity. The conserved amino-terminal domains of the four core histones (H2A, H2B, H3 and H4) contain lysines that are acetylated by histone acetyltransferases (HATs) and deacetylated by histone deacetylases (HDACs). Both acetylation and deacetylation of histones regulates chromatin structure and gene activity.