Vectors were packaged together with the indicated packaging proteins in 293T cells. Supernatants from packaging cells were used for transduction of HT1080 cells. Transduction rates were measured by detection of EGFP by flow cytometry. The cross-packaging efficiency is calculated from the ratio of EGFP-positive cells of the heterologous system versus the homologous system (e.g. 26.4% = 13.5%/51.3%).