Histone H2A combines with Histone H2B to form the H2A-H2B heterodimer. Two H2A/H2B heterodimers interact with an H3/H4 tetramer to form the histone octamer. (1,2) Histone H2A is also modified by various enzymes and can act as a substrate for them. These modifications have been shown to be important in gene regulation.