Similarities and differences also emerge from the comparison of the sumoylation processes of UL44 and PCNA. The most striking similarity is the DNA-dependence of such post-translational modification: in the case of PCNA, clamp loading rather than the mere presence of DNA was shown to be important for stimulation, implying a change in the properties of PCNA upon loading that enhances its capacity to be sumoylated [59] . This could also be the case for UL44: in fact, point mutations preventing DNA binding [8, 9, 36, 60] strongly impaired UL44 sumoylation in cells (Fig. 4C) and abolished the ability of dsDNA to stimulate UL44 sumoylation in vitro (Fig. 4E) . Furthermore, the ability of DNA to stimulate UL44 sumoylation appears to correlate with the efficiency of the UL44-DNA interaction. In fact, addition of dsDNA -for which UL44 has a ,3-to 8-fold higher affinity than for ssDNA [7] -caused a much stronger increase of UL44 sumoylation as compared to ssDNA (Fig. 4A) . In this context, it is important to note that point mutations impairing DNA binding and sumoylation in cells did not compromise the UL44-Ubc9 interaction (Fig. 4D) , suggesting that binding to DNA does not promote UL44 sumoylation by facilitating its binding to Ubc9.