The most important cell lines of biotechnological interest, mouse hybridomas and CHO cells, are known to contain endogenous retroviruses (ERVs) and are known to produce retroviral particles. ERVs exist in 2 forms, a viral form and a proviral form. Retroviral proviruses are transmitted through germ cells and are present in the genomes of almost all vertebrates thus far studied. Humans possess many ERV genomes related to mammalian C type retroviruses and to A, B and D types of this family of viruses. The presence of ERV-like sequences in a cell line to be used in the production of a biological is a potential cause for concern because of the possibility that the endogenous retrovirus may be activated and result in infectious virus being present. particles (IAP) as well as budding C-type particles (Table VI) . With respect to IAPs, Anderson et al. (1990) demonstrated, that the CHO cells' genome contained approximately 300 copies of viral sequence per haploid genome. No intact open reading frame for gag, pol, or env could be detected in clones of either family (Anderson et al., 1990) . In addition, no infectivity has been associated with A-type retroviruses from CHO cells (Kuff and Lueders, 1988; Adamson, 1998) . CHO cells also produce C-type particles at concentrations of <10 3 -10 6 ml −1 . The presence of virus particles was correlated with detectable reversed transcriptase activity (a retrovirus specific enzyme) (Dinowitz et al., 1992) (Table VII) . As for the IAPs, each CHO cell contains between 100 and 300 copies/genome (Dinowitz et al., 1992) . No evidence of infectivity could be detected (Dinowitz et al., 1992; Adamson, 1998) , this may be due to the lack of functional open reading frames, rendering the retroviruses incapable of encoding an intact endonuclease (Dinowitz et al., 1992) .