Because at least some tumour cells are known to contain oncogenes and/or viral genes, there is a theoretical possibility that even small amounts of cellular DNA in a product could transmit genetic information for cancer, or for certain viruses, and thus cause an oncogenic event or pathology related to viral gene expression. The major points supporting such a possibility are that (i) viral nucleic acids can infect cells, (ii) tumour cell DNA can transform 3T3 cells, (iii) viral DNA can infect animals and, (iv) the purified viral oncogene, v-src, caused tumours in chickens. On the other hand, there is no convincing evidence that DNA from tumour cells can cause tumours in animal test systems, and a limited amount of human and non-human primate data are consistent with a level of risk that is not measurable. Estimates of various types of risk from residual cellular DNA suggest that the levels of risk are so low that they are not measurable in most cases.