Having identified Ubc9 as a potential interaction partner of UL44 in Y2H screens, we wished to confirm their physical interaction by an independent experimental approach. For this purpose, pulldown assays with a purified GST-UL44 protein and in vitro-translated, 35 S-labeled Ubc9 were performed. As positive and negative controls, we also assayed the interaction between UL44 and in vitro-translated UL54 or the PB1 subunit of influenza A virus RNA polymerase. As expected, we could detect the interaction of UL54 [14, 29] , but not of PB1, with GST-UL44 (Fig. 1A) . Consistent with the Y2H results, Ubc9 specifically associated with GST-UL44, while no interaction with GST was observed (Fig. 1A) .