UL44, UL54, and Ubc9 proteins were fused to the C-terminus of LexA protein and/or of GAL4 activation domain (GAD). Fusion proteins were then assayed for interaction by qualitative b-galactosidase (b-gal) filter assays and by quantitative b-gal liquid assays. Ubc9, but not in immunoprecipitates obtained from cells expressing only UL44-FLAG (Fig. 3B, right panel) . Similar results were obtained in HeLa cells (Fig. S2) . Altogether, these results demonstrate that Ubc9 can mediate the conjugation of SUMO-1 to UL44 in mammalian cells. Moreover, sumoylation of UL44 in mammalian cells by both SUMO-2 and SUMO-3 could also be detected (Fig. S3) .