The world's first gene-modified pig liver to be transplanted inside a person functioned for 10 days — and was not rejected by the recipient during that time.
The Chinese team behind the transplant, which was conducted a year ago, has just published details of the process in the journal Nature today.
The study is part of a small — but growing — group of animal-to-human whole organ transplants, in a practice known as "xenotransplantation".
Researchers, particularly in the US and China, have pursued successful animal organ transplants in the hope that they could one day address human organ shortages.
Here are five quick questions about the procedure, and medical and ethical issues that must be addressed before xenotransplantation becomes widespread, if it ever does.
What did the transplant involve?
The researchers found that the genetically modified pig liver could function in a human body — at least for 10 days.
The surgery was performed at Xijing Hospital in Xi'an, China, and their subject was an adult patient at the hospital who had been declared brain dead by six independent experts.
After getting family consent, the researchers transplanted a liver they'd removed from a pig into the patient's body.
The pig, a Bama miniature pig, was bred specifically for the purposes of xenotransplantation. The scientists had edited six genes in its genetic blueprint so when its liver was transplanted into a human recipient, the risk of organ rejection was lower.
The surgical team left the patient's original liver in the body, and monitored the new liver for signs of rejection, as well as products a healthy, functional liver was supposed to make, such as bile.
"The liver collected from the six-gene-modified pig functioned very well in the human body," Lin Wang, a researcher at the Fourth Military Medical University in China and study co-author, said.
Professor Wang added the team terminated the experiment after 10 days at the family's request.
While the liver seemed to perform well over the 10-day period, the scientists don't think it's likely the liver could support the human body in the long term.
Instead, they think this research could lead to temporary organ replacements while patients wait for human livers.
Alex Sharland, a researcher in transplantation immunobiology at the University of Sydney who was not involved in the new research, said the study's ability to test how well the liver functioned was "fairly limited" because the patient's original liver had not been removed.
"The authors do acknowledge this themselves," she said.
The short transplant time frame also made it harder to monitor some of the more detailed functions of the liver, or see how it might have behaved over a longer period.
Is this the only pig liver transplant?
No. It's been a big year for the liver xenotransplant field.
In January 2024, a team at the University of Pennsylvania attached a genetically modified pig liver to a patient that had experienced brain death. The liver was used outside of the donated body.
Professor Wang and his team's procedure, done in March 2024, is the first genetically modified pig liver transplant inside a patient.
In May last year, a 71-year-old man in China became the first living person to receive a genetically modified pig liver.
Two weeks after surgery, his surgeon Sun Beicheng said the man was "doing very well", but when the ABC reached out to him about whether the man was still alive, we did not receive a response.
Researchers have been investigating liver xenotransplants for decades. For example, all the way back in 1994, Canadian woman Mavis McArdle was connected to a pig liver outside of her body after she went into a coma.
The pig liver kept her alive for several hours until an appropriate human liver was found.
The difference between the 1994 example and the current spate of transplants is that pigs are now genetically modified to make their organs more humanlike.
What about other types of pig organ transplants?
Livers aren't the only pig organs scientists are trying to adapt for use in humans.
Two men in the US have received genetically modified pig heart transplants since 2022.
The first, 57-year-old David Bennett, survived two months, while 58-year-old Lawrence Faucette survived six weeks.
Genetically modified pig kidneys are also being used, with multiple people having the operation, mostly in the US and China.
The first person to receive a genetically modified pig kidney, US man Rick Slayman, died two months after the procedure.
According to Dr Sharland, hearts and kidneys from genetically modified pigs might be easier to transplant than livers.
"The liver makes thousands of different proteins, and you want them to be compatible with the human system," she said.
"[For this reason,] I don't think [xenotransplantation] is on the horizon for livers, except as a very short-term, temporary procedure."
How ethical are pig organ transplants?
While there have been several cases of experimental whole organ transplants on clinically dead patients, the research still prompts a raft of ethical questions — both with the experiment itself, and the broader matter of xenotransplantation.
Researchers are required to follow strict ethics protocols before starting such work, and the Chinese team ran their pig liver experiment under the supervision of the hospital’s human and animal ethics committees.
They got the informed consent of the patient's four immediate family members before starting the process, and terminated it 10 days later at the family's request.
University of Sydney bioethicist Hojjat Soofi, who was not involved in the trial, said that the fact that the study started after the patient was declared brain-dead complicated matters.
"I think enrolment should have been guided by the patient's prior wishes," Dr Soofi said.
"Such studies should proceed when individuals have explicitly authorised this kind of research in advance."
The short-term nature of the liver transplant also raises questions about the ethics of xenotransplantation, Dr Soofi added.
Xenotransplantation research is often rationalised by the idea that it will help to alleviate human organ shortages.
But short-term transplants don't do this. They just give patients a temporary bridge while they wait for human organs.
"I think we should be more cautious in assessing the potential benefits of xenotransplantation research," Dr Soofi said.
Will xenotransplants solve the organ donation shortage?
Well, it depends who you ask.
Those on the side of organ xenotransplantation believe science is extremely close to genetically modified pig organs being successful.
In February 2025, the US Food and Drug Administration approved the first clinical trial to test the large scale safety of genetically modified pig kidney transplantation.
This means US researchers will no longer have to rely on the small number of patients who have no other medical options available to them, known as "compassionate use cases".
If pig organ transplants do become successful, it could help the thousands of Australians who are either waiting for a transplant or on dialysis.
On the other hand, those against the practice argue that it's a unnecessarily complicated system, and there's a much easier solution: increasing organ donation rates.
There are also potential issues with inter-species diseases. Mr Bennett, the first person to have a genetically modified pig heart transplant, was found to have a pig-derived infection after his death.
The Nature study did investigate disease transmission in their paper, but Dr Sharland "wouldn't feel confident" that there was no possibility of transmission of viruses to the patient.
"I have to say, I don't think we're quite there yet," she said.
