3426 Nucleic Acids Research, 2005, Vol. 33, No. 10 Figure 1. The overall structure of a human NCP. The upper ribbon model is the crystal structure of a human NCP. The arrow above NCP indicates a pseudo 2-fold axis passing through the center of the structure. The bottom figure displays the axial view of the particle. Histone chains are colored blue for H3, green for H4, yellow for H2A and red for H2B. The DNA is shown in ribbon traces for the 146 bp DNA phosphodiester backbones (brown for the I chain and forest green for the J chain). The Mn2+ and Cl ions are depicted by cyan and silver balls, respectively. (11,12) and chicken (13) NCP. The global structure is also similar to that from yeast (14), although the sequence identity is 60–80% between the two species. Therefore, the basic NCP structure is well conserved from yeast to human, despite the considerable differences in the chromatin organization between yeast and higher eukaryotes (32–34). The histone octameric structures of human NCP and other NCPs [2.0 s resolution structure of Xla-NCP (PDB code 1KX3) (11), yeast NCP (PDB code 1ID3) (14) and chicken NCP (PDB code 1EQZ) (13)] were compared by super- imposing residues (H3: 38–134, H30: 38–135, H4: 25–102, H40: 18–102, H2A: 14–118, H2A0: 15–118, H2B: 32–125,