TRANSPLANTATION of organs from one species to another – for example, from pigs to humans. The use of organs from appropriately cloned animals was seen as a possible solution to the shortage of human organs for transplantation. However, research has shown that rejection is a major problem and there is also an unresolved possibility that diseases might be transmitted across the species barrier. It seems likely that STEM CELL research will provide a more realistic source of tissues for transplantation to replace diseased organs in humans.