The demand for human donor organs far outstrips supply. Many patients die as they wait for many years for replacement organs. Animals could provide the answer to the organ shortage, and pigs would make particularly suitable donors because the size and function of their organs closely match those of humans. Moreover, animals could be bred and kept in large numbers. However, the human immune system would recognize a pig's kidney as alien and immediately reject it. For this reason, scientists have begun to modify pigs genetically, to make their cells more human. In order to achieve this it is necessary to clone animals that lack certain genes or have been given additional human genes. However, the aim of changing the surface structure of cells in such a way that the human immune system no longer attacks donor tissue has yet to be achieved.
There is also another danger to transplanting non-human tissue. It is difficult to ensure that there are no retroviruses in the animal's DNA which could become active after transplantation, thus triggering diseases. Pig's heart valves are already being used to replace defective valves in humans, but these are treated prior to use to ensure they contain neither live cells not viruses.