To alter the host range/tissue tropism of retroviral vectors, we produce pseudotype with various alternative envelope proteins as described below for lentivirus. These include the use of VSV-G, RD114, and GALV envelope. The retroviral envelope interacts with a specific cellular protein to determine the target cell range. The retroviral envelope interacts with a specific cellular protein to determine the target cell range. Altering the env gene or its product has proved a successful means of manipulating the cell range. Viruses differ with respect to their tropisms, therefore by replacing the env gene with that of another virus, the host range can be extended, in a technique known as pseudotyping. Vesicular stomatitis virus G protein has been included in Mo-MLV derived vectors (Burns et al, 1994), which are also more stable when purified by ultracentrifugation.