Model of EBV episome segregation onto sister chromatids. Model of EBV episome segregation onto sister chromatids. Clusters of EBNA1 molecules mediate noncovalent association of EBV episomes with cellular chromatin. In S-phase cells, approximately equal amounts of EBNA1 molecules are distributed to sister chromatids while cellular and viral replication are synchronously replicating. Replicated sister EBV genomes are initially catenated, and then get decatenated by the action of topoisomerase II (topo II). Asymmetrically localized EBV episomes in mitotic cells could also be segregated symmetrically during S phase, although dynamic movement of EBV episomes in interphase nuclei is speculative. Teru Kanda et al. J Cell Sci 2007;120:1529-1539 © The Company of Biologists Limited 2007