Simian immunodeficiency virus. Colour transmission electron micrograph of simian immunodeficiency viruses (SIV) in bone marrow culture. The viruses are the small yellow/green bodies with dark centres. The dark centres are nucleoids, which contain the genetic material of the viruses. The culture cell is lying adjacent to a white blood cell, the boundary (red) of which runs diagonally across the image at top left. SIV causes an AIDS- like disease in monkeys. It is thought to be related to the recently discovered, second AIDS (Acquired Immune Deficiency Syndrome) virus, HIV-2 (Human Immunodeficiency Virus type 2). Magnification: x87,500 at 6x7cm size. x300,000 at 8x10"~PORTRAIT"