Mouse leukaemia viruses. Coloured transmission electron micrograph (TEM) of extracellular mouse leukaemia viruses (green). These are retroviruses; they consist of two single-strands of RNA (ribonucleic acid) genome, within a protein envelope. The RNA contains a gene for the enzyme reverse transcriptase. When infecting a host cell this enzyme transcribes the RNA genome into DNA (deoxyribonucleic acid), which is inserted in the host cell's DNA. The virus then uses the cell's machinery to replicate itself. These viruses infect white blood cells. Insertion disrupts the host cell's DNA, often causing the cancer leukaemia, the uncontrollable proliferation of white blood cells. Magnification: x47,500 at 10cm.