Another similarity between UL44 and PCNA sumoylation is that both proteins are modified on canonical (K127 for PCNA, and K410 for UL44) and non-canonical residues (K164 for PCNA, and all other sumoylation sites for UL44, see Table 2 ). However, while PCNA is exclusively sumoylated at the Nterminus, both N-and C-terminal residues of UL44 can be modified. In addition, according to our MS-analysis UL44 can be alternatively sumoylated at 16 different sites, while K127 and K164 appear to be the only target sites in PCNA. This flexibility of UL44 in terms of sumoylation target sites, which is reminiscent of the ones described for Daxx and the small hepatitis delta antigen [57, 58] , is arguably the main difference from PCNA. This makes it extremely difficult to study the physiological importance of UL44 sumoylation, also because mutation of several of these lysines caused protein instability. Currently it is therefore impossible to test if, like in the case of budding yeast PCNA, SUMOmodification of UL44 also plays a role in DNA repair/ recombination [69] .