Although histones are remarkably conserved throughout evolution, several variant forms have been identified. This diversification of histone function is restricted to H2A and H3, with H2B and H4 being mostly invariant. H2A can be replaced by H2AZ (which leads to reduced nucleosome stability) or H2AX (which is associated with DNA repair and T cell differentiation), whereas the inactive X chromosomes in mammals are enriched in macroH2A. H3 can be replaced by H3.3 (which correlates with activate genes and regulatory elements) and in centromeres H3 is replaced by CENPA.