Since our data suggested that UL44 can interact with Ubc9 both in vitro and in yeast cells, we sought to examine whether UL44 and Ubc9 could also co-localize in mammalian cells. To this end, aggregates distributed on a diffuse background fluorescence throughout the nucleus (Fig. 1B) . As previously reported [50] , when expressed alone Ubc9 exhibited a nuclear punctate pattern (Fig. 1B) . Upon co-expression of GFP-UL44 and DsRed2-Ubc9, co-localization of UL44 we analyzed the subcellular localization of GFP-UL44 when transiently expressed either alone or in the presence of DsRed2-Ubc9. As a negative control, we also expressed the DsRed2-UL53 fusion protein, which localizes to the cell nucleus but does not interact with UL44 [32] . In GFP-UL44-transfected cells, UL44 localized in a large number of discrete nuclear with Ubc9 was observed (Fig. 1B) . UL44 also colocalized with a catalytically impaired Ubc9 mutant (Ubc9C93S) [51] , but not with UL53 [32] . In addition, co-immunoprecipitation experiments confirmed that UL44 could physically interact with endogenous Ubc9 in mammalian cells. The specificity of the interaction was confirmed by the inability of UL44 to coimmunoprecipitate with Cyclin D1 (CycD1; Fig. 1C ).