n one ongoing set of experiments, Jeffrey L. Platt at the Mayo Clinic in Rochester, Minn., has created human-pig chimeras by adding human-blood-forming stem cells to pig fetuses. The resulting pigs have both pig and human blood in their vessels. And it's not just pig blood cells being swept along with human blood cells; some of the cells themselves have merged, creating hybrids.

It is important to have learned that human and pig cells can fuse, Platt said, because he and others have been considering transplanting modified pig organs into people and have been wondering if that might pose a risk of pig viruses getting into patient's cells. Now scientists know the risk is real, he said, because the viruses may gain access when the two cells fuse.

However, the best laid plans of men and in this case, pigs, seem headed for disasters of epic proportions if you consider the findings from the Mayo Clinic, where researchers have scrutinised the cells of adult pigs that had received injections of human stem cells during fetal life. Contrary to the expectation that those human stem cells would follow the directive of making target organ tissue, Mayo Clinic find that human DNA has fused with pig DNA in a vast proportion of cells. This means, in those hybrid cells, human DNA and pig DNA are contained in the chromosome as genes.

The pig-human chimaera looks porcine enough. As the stem cell injection was made halfway through fetal life, when the body plan of the developing embryo has already been finalised and put into action, the humanised pig looks exactly like its all-pig cousin.

An organ-say the liver-from this pig will have clumps of human cells, sheets of pig cells and quite a large population of hybrid cells. Hybrid cells may be inheritors of porcine disease. Researchers found them liable to be afflicted by porcine endogenous retrovirus (PERV). This virus is present in all pig cells, but does not affect human cells. This virus, once it appears in hybrid cells, can infect human tissue. This raises the nightmare possibility of a new slow virus infection. (That last one we caught from a chimpanzee has wiped out almost half the globe.)