Histones are the main constituents of the protein part of chromosomes of eukaryotic cells. They are rich in the amino acids arginine and lysine and have been greatly conserved during evolution. Histones pack the DNA into tight masses of chromatin. Two core histones of each class H2A, H2B, H3 and H4 assemble and are wrapped by 146 base pairs of DNA to form one octameric nucleosome. Histone tails undergo numerous post-translational modifications, which either directly or indirectly alter chromatin structure to facilitate transcriptional activation or repression or other nuclear processes. The histone variant H2AX replaces conventional H2A in a subset of nucleosomes. H2AX plays a central role in DNA repair and chromosomal stability and is required for cell cycle progression arrest in response to low doses of ionizing radiation and for efficient repair of DNA double strand breaks.