Originally published 2021-10-21.
In September 2021, surgeons at NYU Langone Health in New York attached a kidney from a genetically modified pig to a brain-dead woman whose family had agreed to the experiment before her life support was withdrawn. For the 54 hours the team watched it, the kidney made urine, brought her abnormal creatinine level back to normal and showed no sign of immediate rejection. Announced in October 2021, it was the first time a pig kidney had worked in a human body without being rejected straight away. It was not yet a transplant into a living patient; that came in 2024.
Below we explain how the experiment worked, why the pig was gene-edited, what the doctors and critics said, and how far pig-to-human kidney transplants have come since.
What the NYU team actually did
The operation was led by transplant surgeon Dr Robert Montgomery. Rather than placing the pig kidney inside the patient's abdomen, the team connected it to blood vessels and kept it outside the body, so they could see it, take samples and react quickly if it began to fail. The recipient was a brain-dead woman who had shown signs of kidney dysfunction, which made it possible to see whether the pig organ could take over the work.
Before asking her family, the hospital consulted medical ethicists, lawyers and religious experts. The family consented in the hope that her death could help others. The study ran for about two and a half days before her ventilator was switched off, as planned.
Montgomery described the result in simple terms: "It functioned normally, and did not appear to be undergoing rejection."
Why the pig had to be gene-edited
The biggest obstacle to animal-to-human transplants, known as xenotransplantation, is the immune system. Pig cells carry a sugar molecule called alpha-gal that humans do not have. Our bodies treat it as foreign and can destroy a pig organ within minutes in what is called hyperacute rejection.
The kidney came from a pig with a single genetic change: the gene that makes alpha-gal had been switched off. These animals, known as GalSafe pigs, were developed by Revivicor, a unit of United Therapeutics. In December 2020 the US Food and Drug Administration approved them both as food for people with a meat allergy linked to alpha-gal and as a possible source of human therapeutics, although any medical use still needs separate approval.
Later experiments, including the living-patient transplants described below, have used pigs with many more genetic edits, some removing other pig genes and adding human ones to make the organ even less visible to the immune system.
Why it mattered
The need for kidneys is enormous. At the time, around 107,000 people in the United States were waiting for an organ transplant, more than 90,000 of them for a kidney, and the average wait for a kidney was three to five years, according to the United Network for Organ Sharing. Many patients live on dialysis in the meantime, and thousands die each year before an organ becomes available. Montgomery told the BBC he believed about 40% of patients waiting for a transplant die before receiving one, and argued that relying solely on human donors would never meet demand.
"The traditional paradigm that someone has to die for someone else to live is never going to keep up," he said. He also pointed out that people already use pigs for food and for medical products such as heart valves and medicines.
A clarification on those heart valves: pig valves have been used in heart surgery for decades, but they are chemically treated so that no living pig cells remain. That is why they do not count as true xenotransplantation and why the NYU experiment, using a living pig organ, was a genuinely new step.
Caution and ethical questions
Not everyone was ready to celebrate. The study involved one patient for three days, which cannot show whether a pig kidney would last for months or years. Brain-dead bodies also differ from living patients in important ways, and many problems, including slower forms of rejection, take weeks to appear.
UK kidney specialist Dr Maryam Khosravi told the BBC's World Tonight programme that researchers had studied animal-to-human transplantation for decades and that the step was interesting, but warned: "Just because we can doesn't mean we should." She said society as a whole needed to settle the ethical questions.
Those questions include animal welfare, the risk that pig viruses could cross into humans, and how to obtain truly informed consent in experimental surgery. An NHS spokesperson stressed that routine pig-organ transplants were still some way off and urged people to record their organ donation decision and tell their families.
What has happened since 2021
Progress has been faster than many expected:
- January 2022: Surgeons at the University of Maryland transplanted a gene-edited pig heart into David Bennett Sr., a living patient. He survived about two months.
- March 2024: Massachusetts General Hospital carried out the first pig kidney transplant into a living person, Richard Slayman, 62. He went home but died some weeks later; doctors said there was no sign the kidney had caused his death.
- April 2024: Lisa Pisano became the second living recipient, at NYU Langone, combined with a heart pump. The kidney was removed after about six weeks because of poor blood flow.
- 2025: More patients, including Tim Andrews, received gene-edited pig kidneys at Massachusetts General. Andrews lived without dialysis for 271 days before the kidney was removed in October 2025, and he later received a human kidney.
- 2025: The FDA cleared formal clinical trials of gene-edited pig kidneys in patients on dialysis, a key step from one-off experiments toward a possible treatment.
What this means for patients
Pig kidneys are still experimental and available only within carefully controlled trials. For people with kidney disease today, the established options remain dialysis and transplants from deceased or living human donors. Anyone wondering whether they might be a candidate for a trial should talk to their doctor or transplant team.
For everyone else, the most useful thing remains the one the NHS highlighted in 2021: register your organ donation decision and let your family know, because human donors are still the main source of life-saving kidneys.
Sources: NBC News, October 2021: U.S. surgeons successfully test pig kidney transplant in human patient; Wikipedia: Xenotransplantation; original reporting by BBC News, October 2021.
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