In January 2022, David Bennett Sr. was out of options. The 57-year-old had been in a Baltimore hospital for more than six weeks, kept alive by a heart-lung bypass machine. He was too sick for the human transplant list and could not get an artificial heart pump because of his irregular heartbeat.
"It was either die or do this transplant," he said before surgery, according to the University of Maryland School of Medicine. "I want to live. I know it's a shot in the dark, but it's my last choice."
On January 7, surgeons at the University of Maryland Medical Center replaced Bennett's heart with one taken from a genetically modified pig. It was the first time a gene-edited pig heart had been transplanted into a living person. The surgery was performed by cardiac surgeon Bartley Griffith. The science that made it possible belonged largely to Muhammad Mohiuddin, a professor of surgery at the medical school and director of its Cardiac Xenotransplantation Program.
From hamsters to baboons
Xenotransplantation (moving organs between species) has been a goal of medicine for more than a century. The reason is simple. More than 103,000 people in the United States are waiting for an organ transplant, and 17 die every day while on the list, according to The Physiologist Magazine. Pig organs are about the right size for people, and pigs can be raised in large numbers.
The problem is the immune system. When a pig organ enters a human body, antibodies recognize sugars on the pig's cells as foreign and attack within minutes.
Mohiuddin has worked on that problem since 1991. He earned his medical degree at Dow Medical College in Karachi, Pakistan, then trained in transplant immunology and surgery at the University of Pennsylvania and other Philadelphia hospitals. In the late 1990s, he was moving hamster hearts into rats and rat hearts into mice. He went on to lead transplant research in a cardiothoracic surgery program at the National Heart, Lung, and Blood Institute in Bethesda before coming to Maryland.
His turning point came in baboons. In a 2016 study in Nature Communications, his team showed that a genetically modified pig heart, attached alongside a baboon's own heart, could keep beating for more than two years when paired with an antibody drug that blocks a key immune signal. That result helped convince the Food and Drug Administration to authorize the first human attempt. "The FDA used our data and data on the experimental pig to authorize the transplant," Mohiuddin said in 2022.
Ten edits
The pig that donated Bennett's heart came from Revivicor, a company in Blacksburg, Virginia. It carried 10 genetic edits. Three genes were knocked out to remove the sugars that trigger rapid rejection. Six human genes were added to help the organ get along with the human immune system. One more gene was knocked out to keep the pig heart from growing too large once inside a human chest.
The heart worked. Bennett showed no signs of acute rejection, and he lived for about two months before his heart failed. Researchers later identified several possible contributors, including a dormant pig virus found in the organ and a treatment that may have contained antibodies against pig tissue. "We have gained invaluable insights learning that the genetically modified pig heart can function well," Mohiuddin said afterward.
A second patient
In September 2023, the team tried again. The recipient was Lawrence Faucette, a 58-year-old Navy veteran from Frederick, Maryland, who had worked as a lab technician at the National Institutes of Health. He was ineligible for a human heart because of other health problems.
"My only real hope left is to go with the pig heart, the xenotransplant," he said before surgery, in a University of Maryland Medical Center release. Two days after the operation he was sitting up in a chair, Scientific American reported. He died about six weeks later, after the heart showed signs of rejection.
Two patients, both deeply ill before surgery, is not much to go on. Mohiuddin has been candid that this is built into the work. "We will never get a perfect patient because a perfect patient will first opt in for a human heart," he told The Physiologist Magazine.
From compassionate use to a trial
Both Maryland surgeries were done under the FDA's compassionate use pathway, which allows experimental treatment for patients with no other options. The next step is a formal clinical trial, and it is now close.
In May 2026, the FDA cleared United Therapeutics, Revivicor's parent company, to begin a trial of its pig heart, called UHeart, in people 50 and older with end-stage heart failure and no other treatment options. The study will start at a single center with up to two patients, each followed for 24 weeks, before it can expand. The pigs carry the same kind of 10-edit design used in Maryland.
The field has moved quickly elsewhere too. Surgeons in Boston and New York have transplanted gene-edited pig kidneys into living patients, and the FDA has cleared early kidney trials as well.
What the patients teach
For Mohiuddin, who was named to Nature's list of 10 people who shaped science in 2022, the goal has always been larger than any single surgery. "If we can provide this option, that will definitely meet the shortage of organs," he told The Physiologist Magazine.
He is also clear that the science is not finished, and that every patient teaches the field something no baboon could. "But we don't know what we don't know," he said, "so that's why we need to follow [patients]."