e106 Nucleic Acids Research, 2007, Vol. 35, No. 16 The models predict that this will be diﬀerent for conformations, which are more dependent on a speciﬁc b sequence, as expected for regular ﬁbers and for some types of double ﬁbers with repeated b sequences. Since these types of ﬁbers have a potential architectural role in the large-scale organization of the chromatin, changes in b within these segments of the ﬁlaments might have fatal eﬀects. An invariable placement of nucleosomes in these regions may therefore be more important than in others. Maintenance of the higher order organization of the genome might thus depend on the existence of series of ‘architectural’ nucleosomes located in invariable positions. Such nucleosomes might possibly belong to the most ﬁrmly bound nucleosomes whose positions are encoded by the DNA (48). ACKNOWLEDGEMENTS I thank Thomas Bentin and Goran Goranovic for critical comments and valuable suggestions in the preparation of this manuscript. 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