Mouse mammary tumour virus (MMTV) particles (red), coloured transmission electron micrograph (TEM). MMTV induces malignant tumours in the mammary glands of certain strains of laboratory mice. The virus replicates in the cells of the glands of lactating mice and is transmitted to their offspring in the milk. MMTV is a retrovirus, meaning its genetic material is composed of ribonucleic acid (RNA) not the usual DNA (deoxyribonucleic acid). Magnification: x12000 when printed at 10 centimetres wide.