It is evident that all biological products derived from cell lines containing endogenous retroviruses or other latent viruses have to be characterized for the presence of virus. In addition, in order to increase their biological safety, first, all biotech products derived from such cell cultures have to be rigorously tested for the absence of retroviral activities/viruses or latent viruses, and second, the purification protocols for biotech products derived from these cells have to be validated for their capacity to eliminate or inactivate retroviruses/latent viruses.