Human foreskin fibroblasts (HFF; from the American Type Culture Collection [ATCC]), HeLa (from ATCC), eco Phoenix (a generous gift from G. P. Nolan, Stanford, USA; [42] ), and Human Embryonic Kidney 293T (HEK 293T; from ATCC) cells were maintained in Dulbecco's modified Eagle's medium (DMEM, Life Biotechnologies) supplemented with 10% fetal calf serum (FCS), 100 U/ml penicillin and 100 mg/ml streptomycin (P/S). COS-1 cells (from ATCC) were maintained in DMEM with 5% FCS and P/S. The U373-SUMO-1 cell line, which constitutively expresses FLAG-tagged SUMO-1 [43] , and the control U373-Neo cell line, stably transfected with an empty vector carrying a Neomycin resistance marker [44] , were kindly provided by G. S. Hayward (Johns Hopkins University School of Medicine, Baltimore, USA) and were maintained in medium containing 0.5 mg/ml Neomycin (G418, Gibco-BRL). HCMV strain AD169 was purchased from ATCC.