To investigate this possibility, we conducted yeast two-hybrid (Y2H) screenings to search for cellular partners of UL44. To our surprise, Ubc9, an enzyme involved in the sumoylation process, was identified as a UL44 protein interaction partner. Sumoylation is a post-translational protein modification analogous to ubiquitination. It consists of reversible and covalent conjugation of SUMO (Small Ubiquitin-related MOdifier) to a protein target [21, 22] . In the sumoylation cascade, the C-terminus of SUMO is activated by an activating enzyme (E1), transferred to a conjugating enzyme (E2, that is Ubc9), and linked to a lysine residue of the substrate protein with the aid of a ligase (E3). Mainly, three SUMO paralogs (SUMO-1, -2, -3) have been identified so far [23, 24] . SUMO-2 and SUMO-3 are highly homologous to one another (95% identity) while they differ from SUMO-1 by 50%. Conjugation of SUMO-1 has been shown to play a functional role in a number of biological processes, ranging from nucleocytoplasmic transport to transcription, the maintenance of genome stability, nucleic acid DNA metabolism, cell signaling, and many others [21] , whereas the role of SUMO-2/23 modification is less clear.