For instance, if a fertilized pig ovum were implanted within a cow's uterus and the resultant piglet successfully brought to term, it's doubtful Halakha would deem it kosher, even though it was produced within a kosher animal and the initial transfer from a non-kosher species was at the microscopic level. Further, in light of the rulings on lactose and casein, generating the isolated proteins of a non-kosher animal instead of the entire animal does not in itself preclude halakhic concern. It seems the only way to exempt transgenic foods is to argue that when foreign genes have become embedded in the host organism's cellular nuclei to the extent that their products are synthesized by its cellular machinery, these products effectively belong to and are characteristic of that organism rather than their source species. However, attempting to conceptually sever the link between a species and its transplanted genes runs afoul of the facts. Consider a virus, a strand of DNA that has no surrounding cell of its own and is dormant until it invades the nuclei of another organism's cells and appropriates the cellular mechanisms.
Many viruses even insert themselves into the host's DNA strand.
Only within and by virtue of the host's cells can a virus synthesize its proteins. Yet, it would strain the bounds of logic to argue that these proteins are those of the host organism. Although they are produced by genes operating within the organism's cellular nuclei and making use of its cellular resources, they do not belong to it. They are debilitating and often life-endangering to the host, and its immune system recognizes them as a foreign threat and strongly counteracts them. Therefore, we can only deny the connection between a species and the products of its transplanted genes if we are also prepared to deny the concept of viral infection.