Transducing lentiviral vectors that transfer the exogenous gene(s) into target cells are typically defective for viral replication because at least some of the trans-acting sequences encoding the viral proteins have been deleted. To propagate vector virus, the viral proteins are supplied either by transient transfection of plasmids expressing the viral proteins or by using packaging cell lines, which contain the viral expression plasmids stably integrated into their cell genome (1). Packaging cells provide a safety advantage for propagating retroviral vector virus because they reduce the chance of recombination that might result in the production of replication-competent (RC) virus. During development of the packaging cells, the expression plasmids can be introduced sequentially at different times so that they will be located at different places in the cell genome. In addition, avoiding cotransfection of all of the trans-acting components simultaneously, a recombinagenic step (2), further reduces the likelihood of recombination.