DR KLAUS BOLLER / SCIENCE PHOTO LIBRARY DR KLAUS BOLLER / SCIENCE PHOTO LIBRARY
Porcine endogenous retrovirus (PERV). Coloured transmission electron micrograph (TEM) of PERV particles or virions (red) in a culture of human kidney cells. At left is a virion during the process of infection, while at right a virion buds from its host cell. PERV is normally present in pigs as a stretch of DNA integrated into the host's genetic material, and is inherited like a normal gene. Only rarely does this form produce viral particles. Although PERV does no harm to pigs, the discovery that it can infect human cells in culture has led to concern that it might cause disease when pig organs are transplanted into humans. Magnification unknown.
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