Xenotropic murine leukaemia virus particles (brown), coloured transmission electron micrograph (TEM). Each virion (virus particle) consists of a core of RNA (ribonucleic acid) surrounded by a protein nucleocapsid and lipid envelope. This mouse retrovirus is also able to infect cells of other species, including humans. Magnification: x65,000 when printed at 10 centimetres tall.