Advances in controlling immunological rejection have raised the possibility that pigs could be used as a source of organs and tissues for transplantation into humans,. However, the report that one pig kidney cell line, PK15, produces Ctype retroviruses capable of infecting human cells has reinforced fears over the potential risks of viral infections associated with xenotransplantation,. Further support for these fears comes from the discovery of two different classes of porcine endogenous proviruses (PERVs), capable of infecting human cells, in PK15 cells as well as in a variety of normal porcine tissues.