H2A molecules are mostly conserved among variants. The variability in gene expression exists among the regulatory machinery that manages H2A expression. Researchers studied eukaryotic evolutionary lineages of histone proteins and found diversification among the regulatory genes. The greatest differences were observed in core histone gene cis-regulatory sequence motifs and associated protein factors. Variability in gene sequence was seen in bacterial, fungi, plant, and mammalian genes. One variant of H2A protein is H2ABbd (Barr body deficient) variant. This variant is composed of a different genetic sequence compared to H2A. The variant functions with transcriptionally active domains. Other variations associated with H2ABbd are located within its C-terminus. H2ABbd has a shorter C-terminal domain compared to the large C-terminal found on H2A. The two C terminals are about 48% identical. H2ABbd functions with active chromosomes. Thus far, it is missing from Xi chromosomes in fibroblast cells. Lastly, it found to be associated with acetylated H4. Different functions of H2A.Z compared to H2A are correlated with genetic differences between H2A and the variant. Resistance to nucleosomes occurs in H2A.Z by binding to H1 factor. H2A.Z gene is an essential gene in yeast and it is denoted as Htz1. Comparatively, vertebrates have two H2A.Z genes. These genes, H2A.Z1 and H2A.Z2 encode for proteins that differ from H2A.Z by three residues. At first researchers figured that these genes were redundant; however, when a mutant H2A.Z1 was created, it resulted in lethality during mammalian tests. Therefore, H2A.Z1 is an essential gene. On the other hand, researchers have not identified the function of H2A.Z2 variant. It is known that it is transcribed in mammals and this gene expression is conserved among mammalian species. This conservation suggests that the gene is functional. When studying H2A.Z in plants species, the protein different among residues from species to species. These differences contribute to differences in cell-cycle regulation. This phenomenon was only observed in plants. Phylogenetic trees were created to show the divergence of variants from their ancestors. The divergence of variant, H2A.X, from H2A occurred at multiple origins in a phylogenetic tree. Acquisition of the phosphorylation motif was consistent with the many origins of H2A that arose from an ancestral H2A.X. Finally, the presence of H2A.X and absence of H2A in fungi leads researchers to believe that H2A.X was the original ancestor of the histone protein H2A Modification of H2A H2A modification is under current research. However, modification of H2A does