Retrovirus vectors have been used for a variety of applications requiring gene transfer, including the production of transgenic animals. In all such cases, the transgene is delivered through an infectious particle and as part of a retrovirus genome. In almost all cases, the retrovirus genome is replication defective because its genes have been replaced with the transgene. Following infection, the viral genome containing the transgene is reverse transcribed, and the resulting double-stranded DNA copy integrates into the host cell genome as a provirus, which functions thereafter as a host cell gene.