Having demonstrated that UL44 is sumoylated by Ubc9 in vitro and in transfected cells, we sought to investigate whether a similar modification also occurs naturally in HCMV-infected cells. Protein lysates of HFFs infected with HCMV and collected at different times post-infection (p.i.) were analyzed by western blotting with an anti-UL44 antibody. Two main bands of 65 and 80 kDa were observed above the primary UL44 band of 50 kDa during the whole time course of lytic infection, being detectable already at 24 h p.i. (Fig. 5A) . A third band of ,95 kDa was also visible from 48 h p.i. These subforms were similar in electrophoretic mobility to the UL44 bands covalently modified by SUMO-1 in transfected cells (Fig. 3A) . A densitometric analysis of the protein bands (Fig. 5B ) revealed that the relative amount of the sumoylated forms increased during the course of HCMV infection, becoming ,50% of total UL44 protein at 120 h p.i.