Histones are basic nuclear proteins that are responsible for the nucleosome structure of the chromosomal fiber in eukaryotes. Nucleosomes consist of approximately 146 bp of DNA wrapped around a histone octamer composed of pairs of each of the four core histones (H2A, H2B, H3, and H4). The chromatin fiber is further compacted through the interaction of a linker histone, H1, with the DNA between the nucleosomes to form higher order chromatin structures. Global mass spectrometric analysis of histone H2A variants/subtypes and their modifications have only recently been carried out. Nine histone H2A subtypes have been identified, including H2A2A and H2A2B. The two main H2A variants, H2AO and H2AC, as well as H2AL, were either acetylated at Lys 5 or phosphorylated at Ser 1. For the replacement histone H2AZ, acetylation at Lys 4 and Lys 7 was found.