Many aspects of chromosome biology are fundamentally linked to events that occur during the DNA synthesis (S) phase of the cell cycle. The DNA must be duplicated once, and once exactly, each S phase. Local chromatin structure must also be re-assembled each S phase to incorporate newly replicated sister chromatids. The replication fork is the one complex that potentially interacts with every nucleotide of the genome, providing a mechanism to couple chromatin assembly to S phase passage.

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... ly, eukaryotic genomes contain regions of structurally distinct chromatin, such as heterochromatin, defined by distinct patterns of histone modification and specific protein associations. 1 Heterochromatin is generally associated with repeated sequence elements near centromeres, telomeres and other sites. Evidence suggests that heterochromatin assembled during S phase supports the association of multiprotein complexes required for many chromosome transactions, including transcriptional silencing, sister-chromatid cohesion, and kinetochore function. These complexes are in turn essential for regulated gene expression, equal chromosome segregation and genomic stability. Intriguingly, recent studies indicate that these processes are linked to S phase by temporal mechanisms as well as by replicationdependent activities (Fig. 1) .