As mentioned, xenotransplantation was traditionally conceived of transplanting animal organs into human beings. Such interspecies transplants, though, impose even greater risks than allotransplantation (human-to-human transplants) on the organ recipient because animal organs tend to induce a much more extreme immune response than do human organs. Recent research attempts to genetically engineer the animals to remove the most immunogenic components. But some researchers are exploring the possibility of going further by genetically engineering the animal so that it cannot produce the desired organ on its own. This creates an unoccupied niche in the animals. Human pluripotent stem cells are then inserted into the animal to occupy that niche, with the result that the relevant organ is made entirely of human cells (Waltz 2017.)