The nucleosome is made up of four core histone proteins (H2A, H2B, H3 and H4) and is the primary building block of chromatin. The N-terminal tail of core histones undergoes multIPPle different post-translational modifications including acetylation, phosphorylation, ubiquitination, methylation and ADP-ribosylation. These modifications occur in response to cell signal stimuli and have a direct effect on gene expression. The role of histone variants, and specially those of H3 and H2A, in various nuclear processes has been long appreciated. There are at least three different families of H2A variants present in a variety of organisms from yeast to mammals, and the degree of conservation among members of each family is greater than that of the canonical H2A. H2AX is thought to play a role in DNA double-strand break repair, the serine in the SQEY motif of H2AX is phosphorylated at the site of the DNA damage and serves as a signal for the recruitment of repair proteins. Macro H2A1, another H2A variant, has been shown to have a role in X-chromosome inactivation and dosage compensation in mammals, where it is found to localize to the inactive X after silencing has been established.Synonyms: H2A Histone Family, HIST1H2A