MLV infected rat fibroblast cells. Coloured scanning electron micrograph (SEM) of rat fibroblast cells infected with the murine, leukaemia virus (not visible).The murine leukemia viruses (MLVs or MuLVs) are retroviruses named for their ability to cause cancer in murine (mouse) hosts. Some MLVs may infect other vertebrates. Applications include gene therapy: MLV-derived particles can deliver therapeutic genes to target cells, cancer studies: MLVs are used to study cancer development and as a model retrovirus in viral clearance studies. Magnification x 3000 at 10cm wide. Specimen courtesy of Greg Towers, UCL. Specimen courtesy of Professor Greg Towers, University College London, UK