 There are several types of covalent histone modifications, which commonly involve lysine residues found in the protruding tail of histones.Modifications often include the addition or removal of an acetyl (-CH3CO) forming acetyllysine or reverting to lysine, up to three methyl (‑CH3), ubiquitin or a sumo protein group.The various modifications have downstream effects on gene regulation, in which genes can be activated or repressed.