Lentiviral particles were produced by transient transfection of HEK 293T cells with packaging plasmids helper D8.9 (Addgene) and helper Ampho (kindly provided by the Tissue Culture Facility at the IEO, Milan, Italy) and either pTRIPZ-shUBC9 vector (Open Biosystems) or non-silencing pTRIPZ control vector (Open Biosystems) by the calcium phosphate precipitation method. Supernatants were collected at 48 h post-transfection and used for the transduction of U373-SUMO-1 and U373-Neo target cells in the presence of polybrene (8 mg/ml, Sigma). Selection was done in puromycin (0.5 mg/ml, Invitrogen) for two weeks prior to 4-day doxycycline (1 mg/ml, Sigma) induction to obtain Ubc9 silencing. Ubc9-knocked-down cells were screened for Red Fluorescent Protein expression and used for further experiments.