Murine leukaemia virus. Coloured scanning electron micrograph (SEM). Murine leukaemia viruses (MLV) (red particles) are gamma retroviruses that cause cancer in mice. There are no known gamma retroviruses that infect humans. Studying gamma retroviruses and how they cause cancer led to the discovery of oncogenes and the viral enzyme reverse transcriptase and a series of Nobel prizes. Specimen courtesy of Professor Greg Towers, University College London. Magnification: x4000 when printed 10cm wide