VeroE6 cells (ATCC CRL-1586), 293 cells (ATCC CRL-1573) and A549 cells (ATCC CCL-185) were maintained in Dulbecco's modified minimum essential medium (DMEM) containing 10% fetal calf serum (FCS). BHK/T7-9 cells that stably express T7 RNA polymerase [79] were maintained in MEM-alpha containing 10% FCS. Penicilin (100 U/ml) and streptomycin (100 g/ml) were added to the culture media.