Since sumoylation mainly occurs in the nucleus of mammalian cells [56] and UL44 is translocated to the host cell nucleus during HCMV infection [11] , we decided to investigate whether nuclear localization might be a prerequisite for conjugation of SUMO-1 to UL44. We therefore analyzed the ability of UL44bDNLS, a derivative of UL44 bearing point mutations within the nuclear localization signal (NLS, 425 PNTKKQK 431 ) which prevent UL44 nuclear accumulation [11] , to be modified by SUMO-1. Interestingly, mutations of UL44 NLS impaired the sumoylation of a V5-UL44 fusion protein (V5-UL44DNLS, Fig. 3C ), suggesting that SUMO-1 modification of UL44 most likely occurs into the nucleus or during nuclear import.