Scientists, including Sachs, are still tinkering with better ways to trick the primate immune system into accepting pig tissue. They ultimately want to find a protocol that minimizes the use of immunosuppressants, which raises the risk of infection. (Two of the eight baboons in Sachs' last transplant experiments died of infections.)

One option, says Sachs, is to transplant bone-marrow cells along with the kidneys and thymus, which in some cases has been shown to improve tolerance for transplanted organs. Scientists are also developing additional genetic modifications that might improve the long-term survival of the organ, such as adding human genes for proteins involved in immune regulation or blood clotting.

Sachs says the team hasn't yet decided on the exact measures of success to achieve in baboons before starting human trials. Ethical and safety discussions will likely be required to design a human study. The researchers are carefully monitoring potential viruses that might be passed between pigs and baboons or humans, which has been a major health concern with xenotransplantation.

Sach's most recent research, though, published last month in the journal Transplantation, could be good news for people who are highly sensitive to proteins in human tissue, and are therefore much less likely to match organs from a human donor (about 20 percent of people on the waiting list for kidney transplants). By exposing blood from such patients to tissue from genetically modified pigs, Sachs and team found that the individuals' hypersensitivity did not extend to pigs. Given their low chance of finding suitable human organs, these people might be the first candidates for pig-to-human transplants.

Platt says that transplants from pigs might actually be safer than human-to-human transplants in the long run. "You can screen the organ in advance for infectious agents and other problems," he says, a process for which there isn't much time in today's urgent human transplants. And, he adds, because you can schedule the surgery in advance, you can plan pretreatments, such as bone-marrow transplants, that might better prepare the patient's immune system for the foreign organ.