MMLV integrates into the host genome and exists in infected cells as a DNA provirus. Cell division is required for infection. Virus is not lytic. The data suggests a pathogenic mechanism in which chronic productive retroviral infection allowed insertional mutagenesis resulting in cell transformation and tumor formation. The host range of recombinant MMLV vectors depends on the specificity of the viral envelope. The ecotropic env gene produces particles that only infect rodent cells. The amphotropic env gene allows infection of rodent and non-rodent cells, including human cells. VSV-G envelope allows infection in a variety of mammalian and non-mammalian cells. Recombinant MMLV retroviral vectors serve as efficient viral vector tools for introducing genes permanently into a wide variety of dividing cells. This retroviral system came into prominence as a gene delivery method to generate iPS cells.