Schematic of approach to generating recombinant viruses. Expression cassettes were created in the pGL3-Basic vector (Promega) by insertion of the EBV vIL-10 gene along with the MHV76 gp150 promoter (Pgp150). Targeting cassettes were created by insertion of the expression cassette between the MHV76 terminal repeat segment (TR) and unique sequences of the left hand end (LHE, prototype orientation) in the pBSLHE-TR vector (see Methods). The targeting cassette was generated for this study with transcription from Pgp150 in the leftward direction, as determined by the asymmetric regeneration of the BamHI site during insertion (X = no restriction site). The targeting cassette and MHV76 DNA were co-transfected into NIH-3T3 cells for recombination. Once isolated, recombinant viral DNA was co-transfected along with the TR-LHE segment from pBSLHE-TR into NIH-3T3 cells to generate revertant viruses.