Production of retroviral vector particles. This figure illustrates the concept of the strategy used to derive retroviral vector preparations free of replication competent virus. The diagram at the top shows the organization of a retroviral genome. Murine oncoretroviruses have coding sequences for matrix proteins (GAG), reverse transcriptase (POL), and envelope proteins (ENV). To reduce the risk of recombination and generation of replication competent particles, these coding sequences are separated on 2 or more expression cassettes before transfer into a tissue-culture cell line. The coding sequences for viral proteins are replaced by the coding sequences for the intended therapeutic protein in the vector. Various strategies are used for introducing the vector genome into the packaging cell, which initiates production of replication defective retroviral vector particles that can be used to introduce the therapeutic transgene into target cells.