pYD-C235 is a pLPCX-derived retroviral vector (Clontech) that expresses a DsRed gene driven by an internal ribosome entry site 2 (IRES2) [45] . pYD-C474 was created by PCR amplifying the coding sequence of the pGFP-UL21a fusion protein from pADgfpUL21a (see below) and ligating it into the multiple cloning site of pYD-C235. pYD-C580 was created by replacing the coding sequence of wild-type UL21a in pYD-C474 with that of mutant UL21a carrying two stop-codon mutations at the N-terminus (i.e. UL21a stop ) [30] . Vectors expressing pGFP-UL21a truncation mutants were derived from pYD-C235 while vectors expressing point mutants were derived from pYD-C474. Truncation mutants were made by PCR amplifying the targeted UL21a coding sequences and point mutants were created using a QuickChange XL kit (Stratagene). Primers used to create these mutants are listed in Table S2 . pYD-C160, pYD-C175, and pYD-C682 are pRetro-EBNA derived retroviral expression vectors that expressed GFP, UL21a, and UL21a stop , respectively. pYD-C648 and pYD-C649 are pLKO-based lentiviral vectors expressing GFP-TetR and carrying the CMV-TetO 2 promoter, respectively (generous gifts from Roger Everett, University of Glasgow Centre for Viral Research) [46] . YD-C665, YD-C667, and YD-C669 are lentiviral expression vectors created by cloning the UL21a stop , UL21a PH-AA , and UL21a PR-AA sequences into the multiple cloning site of YD-C649. To produce pLKO-based lentiviruses, 293T cells were transfected with corresponding pLKO vectors along with packaging plasmids. Lentivirus was collected at 48 and 72 hours and used to transduce MRC-5 cells. To create GFP-TetR expressing stable cells, Hela cells were transduced with pYD-C648 derived lentivirus and sorted for GFP expression 48 hours later. GFP-positive cells were collected, grown in the presence of G418 (500 mg/ml), and frozen as cells stably expressing GFP-TetR. These stable cells were then transduced with lentivirus derived from YD-C665, YD-C667, and YD-C669, selected with puromycin (2 mg/ml), and tested for tetracycline (1 mg/ml)-regulated expression of targeted genes.