In addition to protein degradation, ubiquitin-modification has been suggested to be involved in the regulation of protein function. It was reported that monoubiquitination of the Gag protein of retroviruses is required for virus budding [22, 24, 25] . Depletion of free ubiquitin by proteasome inhibitors prevents Gag ubiquitination, subsequently blocks virus progeny release/budding. In addition, ubiquitination of human immunodeficiency virus type 1 Tat protein and human T-cell leukemia virus type 1 Tax protein has been shown to modulate their transactivation activities [20, 23] . We speculate that monoubiquitination is also an important machinery for post-translational modification and activation of CVB3 viral proteins. In the current study, we have shown that CVB3 RNA-dependent RNA polymerase 3D is posttranslationally modified by ubiquitination, suggesting a critical role of protein ubiquitination in the regulation of viral protein functions.