Top Down analysis revealed that at least fourteen genes encoding histone H2A are 
coexpressed in HeLa cells. Characterization of these species revealed that all 
except H2A.Z and H2A.F/Z were alpha-N-acetylated, H2A.O and H2A.C,D,I,N,P were 
the most abundant, and those exceeding approximately 10% abundance lacked 
post-translational modifications. This unequivocal identification of H2A forms 
illustrates the advantages of Top Down Mass Spectrometry and provides a global 
perspective of H2A regulation through the cell cycle.