Crystal packing A notable difference between the human and X.laevis NCP crystal structures is the molecular arrangement (Figure 4), which is associated with the tighter packing within the human crystal. As reported previously (35), the packing of nucleosomes within a crystal does not necessarily reﬂect the molecular arrangements in the condensed chromatin. How- ever, it is still interesting to examine the packing differences in terms of the internucleosome positioning. For instance, the Xla-NCP structure revealed that the histone H4 tail contacts H2A/H2B of a neighboring particle (11,12). A recent study, using disulﬁde cross-linking between these histones, found that the chromatin ﬁber comprises two stacks of nucleosomes, in accord with the two-start model (36). It is thus interesting to investigate human NCP arrange- ments in the crystal lattice in comparison with other crystals. Nucleic Acids Research, 2005, Vol. 33, No. 10 3429 Remarkable differences were observed in the DNA–DNA con- tacts between human NCPs. The DNA duplex at SHL 2 (Figure 4A, orange circles) collides with the DNA at SHL 5 (Figure 4A, pink circles) of the adjacent particle. This collision causes the tighter and looser twist in each DNA path (Figure 4B and C, respectively). In detail, it is likely that this inter-particle contacts occur through water-mediate interactions between the cytosine base at the location 51 and the phosphate group at the location 18 and between the two phosphate groups at the location 17 and 51 (Figure 5A). The similar groove–backbone interaction was observed in the crystal packing of DNA duplex dodecamers (37,38). In this packing, the duplexes were adjusted to lie close together by the reciprocal ﬁt of the sugar–phosphate backbone into the major groove. Therefore, it is also likely that the self-ﬁtting of DNA molecules by these interactions modiﬁes the DNA structure within the tighter packing of human NCP.