The VSV-based vaccine expressing MARV GP has also been demonstrated to be effective as a post-exposure treatment. A hundred percent survival of NHPs was observed when the vaccine was administered 20 to 30 minutes after MARV infection [266] . Delaying the time before treatment results in incomplete protection, although five of six animals or two of six animals still survived when given the treatment 1 or 2 days after MARV exposure, respectively [205] . These post-exposure treatments may be useful to prevent disease after known exposure to MARV, such as a laboratory accident, but the effective time frame during an outbreak might be too short and alternatives are needed. The Human Cytomegalovirus DNA Polymerase Processivity Factor UL44 Is Modified by SUMO in a DNA-Dependent Manner During the replication of human cytomegalovirus (HCMV) genome, the viral DNA polymerase subunit UL44 plays a key role, as by binding both DNA and the polymerase catalytic subunit it confers processivity to the holoenzyme. However, several lines of evidence suggest that UL44 might have additional roles during virus life cycle. To shed light on this, we searched for cellular partners of UL44 by yeast two-hybrid screenings. Intriguingly, we discovered the interaction of UL44 with Ubc9, an enzyme involved in the covalent conjugation of SUMO (Small Ubiquitin-related MOdifier) to cellular and viral proteins. We found that UL44 can be extensively sumoylated not only in a cell-free system and in transfected cells, but also in HCMV-infected cells, in which about 50% of the protein resulted to be modified at late times post-infection, when viral genome replication is accomplished. Mass spectrometry studies revealed that UL44 possesses multiple SUMO target sites, located throughout the protein. Remarkably, we observed that binding of UL44 to DNA greatly stimulates its sumoylation both in vitro and in vivo. In addition, we showed that overexpression of SUMO alters the intranuclear distribution of UL44 in HCMV-infected cells, and enhances both virus production and DNA replication, arguing for an important role for sumoylation in HCMV life cycle and UL44 function(s). These data report for the first time the sumoylation of a viral processivity factor and show that there is a functional interplay between the HCMV UL44 protein and the cellular sumoylation system. Most replicative DNA polymerases include a catalytic subunit, responsible for DNA polymerization, and a processivity factor that holds the catalytic subunit on DNA to allow continuous DNA synthesis. One of the best-studied processivity factors is proliferating cell nuclear antigen (PCNA) of eukaryotic DNA polymerases d and e [1] . PCNA, which belongs to the family of so-called ''sliding clamps'', has no inherent DNA-binding capacity, but with the aid of clamp loader proteins is assembled onto DNA as a toroidal homotrimer [2] . In addition to DNA replication, PCNA has been implicated in DNA recombination and repair, as well as in DNA methylation, chromatin remodeling, and cell cycle regulation [1] . Consistent with its pleiotropic functions, it interacts with a plethora of proteins [3] and undergoes a number of posttranslational modifications, including phosphorylation, acetyla-tion, ubiquitination and sumoylation, which are believed to regulate its subcellular localization, stability and protein binding specificity [4, 5, 6] .