Given the huge shortage of donor organs, researchers have been trying to find ways to transplant animal organs across different species (known as ”xenotransplantation”), with the eventual aim of transplanting animal organs into humans. The major stumbling block, says Dr Muhammad Mohiuddin (US National Heart, Lung, and Blood Institute) in a paper in PLoS Medicine, is that the immune system in the animal receiving the organ tends to reject the transplant.