Bill Stewart spent over two years tethered to a dialysis machine, his life slowly shrinking into a pattern of medical routines. His kidneys, worn down by years of high blood pressure, had failed him. Like more than 800,000 Americans with end-stage kidney disease, he joined the waitlist for a transplant—a list where time often runs out before a donor is found.
Then, on June 14, at Massachusetts General Hospital, Stewart received an organ that was once inconceivable: a kidney from a genetically modified pig.
“It’s been a fantastic thing,” Stewart told CNN, reflecting on his return to work. “Just to be outside and have the procedure behind me and know that there’s hopefully a bright future ahead—that’s been great.”
Stewart’s transplant marks a turning point in the field of medicine. This summer, the FDA gave its approval for the first-ever human clinical trials using gene-edited pig kidneys. The company behind Stewart’s transplant, eGenesis, now plans to treat 30 patients over the next two and a half years. Another biotech company, United Therapeutics, is preparing its own trial. Together, they are opening a new frontier in organ transplantation—for a future where no one dies waiting for a kidney.
Making Pig Organs Human-Friendly
The idea of transplanting animal organs into humans, what scientists call xenotransplantation, has long felt like science fiction. But recent advances in gene editing have made it all real.
At the heart of the eGenesis approach is CRISPR, a powerful tool for precisely editing the genome. Their scientists used it to remove a pig gene that produces a carbohydrate called alpha-gal. In humans, alpha-gal can trigger an aggressive immune response, making the foreign organ unviable. Without it, the kidney is more likely to survive in a human host.
Dr. Robert Montgomery — the lead doctor in charge of the surgery — himself received a heart transplant years ago. He has since helped pioneer some of the earliest xenotransplant procedures in humans, including into patients who were brain-dead, to better observe the organ’s performance.
Yet these early procedures were limited. Many involved patients who were critically ill and had no other options. “This next study allows us to study this in more patients and get a better sense of how generally applicable this technology is going to be,” said Mike Curtis, president and CEO of eGenesis.
Now, the goal is to move from emergency, one-off surgeries to rigorously controlled trials. Only patients 50 or older, with end-stage kidney disease and dependent on dialysis, will be eligible. These patients must also be on the transplant waitlist—a list that, as of this writing, includes about 86,000 people waiting for a kidney.
The Pioneers
Tim Andrews, 67, was the first patient in the eGenesis program. He received his pig kidney in January 2025. More than seven months later, he is still living without dialysis. He now holds the record for the longest-surviving recipient of a gene-edited pig organ.
The very first pig kidney transplant at Mass General occurred in March 2024. The recipient, 62-year-old Rick Slayman, died two months later—not due to the organ failure, but to unrelated cardiac issues. Other patients have seen their transplanted organs removed after infections or immune complications.
Still, the field is progressing quickly. At NYU Langone, doctors have performed several pig kidney transplants under compassionate use protocols, where standard treatments had failed. In April 2024, 54-year-old Lisa Pisano received both a mechanical heart pump and a pig kidney. The kidney was removed a month later, and she passed away in July. Another patient, Towana Looney, lived dialysis-free for 130 days before an unrelated infection required the kidney’s removal.
Each of these cases added to the growing body of knowledge. But they also made something clear: for this technology to work, patients need to be well enough to handle it. “Researching this technology in people who are healthier, who are not so far along in the course of renal disease, will also be key,” said Dr. Leonardo Riella, a transplant nephrologist at Mass General.
A Future Beyond the Waitlist
The U.S. transplant system is buckling under pressure. Over 100,000 Americans are waiting for an organ. Most—about 86%—need a kidney. The average wait time is three to five years, but for people like Stewart, who has Type O blood, it can stretch to a decade.
This delay is deadly. The five-year mortality rate for people on dialysis is over 50%.
Patients seem ready. A recent survey by the American Association of Kidney Patients found that over 70% would accept a pig kidney if it were FDA-approved. “Advances in xenotransplantation are giving our community hope,” wrote Kevin Longino, CEO of the National Kidney Foundation, in a press release. “It is gratifying to know they’re being heard.”
Stewart heard the call early. He describes himself as “a little bit of a science nerd.” Before agreeing to the transplant, he consulted with Andrews, the first eGenesis recipient. “Worst case scenario, they can always take it out,” he said.
His gamble paid off. A week after the surgery, Stewart walked out of the hospital. He’s now back at his two jobs, one of which involves working with people with disabilities. He has returned to his favorite trails with his wife, slowly rebuilding stamina.
Transplants Without Borders
The pig kidneys used in these procedures come from animals bred in biosecure facilities. Their genes have been edited to reduce immune rejection and inactivate porcine endogenous retroviruses—remnants of viral DNA in the pig genome that could pose a risk to human recipients.
None of the recipients so far have shown signs of viral transmission. Still, that risk is being closely monitored.
The pig kidney Stewart received is a specific version called EGEN-2784, developed by eGenesis. If the trial proceeds smoothly, it could become the first gene-edited pig organ to receive FDA approval for general use.
Even if a pig kidney provides just one or two years of dialysis-free life, it could buy patients valuable time. “That’s already a huge advantage,” Riella said. “A year, hopefully longer than that.”
Meanwhile, United Therapeutics plans to begin its own trial this year with up to 50 patients. For the first time in medical history, two concurrent trials of xenotransplants are underway in humans.
Back in New Hampshire, Stewart visits his old dialysis clinic—not as a patient, but as proof of what’s possible. “Let everyone know I’m doing all right,” he said, “and maybe kind of give some people some hope.”
