Histones are basic nuclear proteins that are responsible for the nucleosome structure of the chromosomal fiber in eukaryotes. Two molecules of each of the four core histones (H2A, H2B, H3, and H4) form an octamer, around which approximately 146 bp of DNA is wrapped in repeating units, called nucleosomes. H2A.X, also known as H2AFX, is a variant of Histone H2A that is found in a subset of nucleosomes. H2A.X has a unique C-terminal tail that becomes phosphorylated following double-stranded DNA breaks following irradiation or apoptosis. Phosphorylation of H2A.X on Ser139 is one of the earliest markers for apoptosis and DNA damage.