Update (October 31): According to an abstract to be presented November 5 at a meeting of the American Heart Association, the electrical activity of the transplanted pig heart, as measured by EKG, differed from expected patterns. STAT’s Morning Rounds newsletter notes that the finding “raises questions for the field of xenotransplantation.”

Update (May 5): MIT Technology Review reports that the heart David Bennett received was infected with porcine cytomegalovirus, a factor that may have contributed to his death.

Update (March 9): Transplant recipient David Bennett died yesterday, two months after his surgery, the University of Maryland Medical Center has announced. According to the university’s statement, the transplanted pig heart performed well for several weeks after the surgery with no signs of rejection, but Bennett's condition began deteriorating several days ago. “We remain optimistic and plan on continuing our work in future clinical trials,” surgeon Muhammad Mohiuddin says in the statement.

On Friday, January 7, a Baltimore man became the first person to have his heart replaced with that of a genetically modified pig, the University of Maryland Medical Center announced today. The procedure, given the go-ahead by the US Food and Drug Administration (FDA) on December 31, was the first in decades to attempt transplanting a heart into a human from another species, and the first such operation ever to use a heart from a pig engineered to avoid rejection by the human immune system. While the patient, David Bennett, Sr., was still attached to a heart-lung bypass machine today, he is doing well, his doctors tell The New York Times.

“This is a truly remarkable breakthrough,” Robert Montgomery, a transplant surgeon at NYU Langone, tells USA Today. “I am thrilled by this news and the hope it gives to my family and other patients who will eventually be saved by this breakthrough.”

See “Replacement Parts

The transplantation field has long been dogged by a shortage of human organs available for transplantation, with many patients spending years on waiting lists as their health declines. In the past, doctors have attempted transplanting organs from animals such as baboons and chimpanzees into humans, but recipients did not survive for long. With the rise of gene editing, researchers began to tinker with the genes of pigs to make their organs more palatable to human immune systems, and several years ago tried cross-species transplantation from modified pigs to baboons. A few months ago, Montgomery successfully transplanted a modified pig kidney into a person, but in that case the recipient had no detectable brain activity, and the procedure was performed experimentally rather than as a therapy.

According to the Times, the pig that provided Bennett’s new heart had four genes knocked out and six human genes added, all with the aim of preventing immune rejection. An additional gene was knocked out to prevent the heart from continuing to grow post-transplantation.

Bennett’s son, David Bennett, Jr., tells the Times that he was incredulous at first when his father explained that he would be receiving a pig heart. “He’d been in the hospital a month or more, and I knew delirium could set in. I thought, no way, shape or form is that happening.”

So far, the heart is “working and it looks normal,” Bartley Griffith, the surgeon who performed the operation, tells the Times. “We are thrilled, but we don’t know what tomorrow will bring us. This has never been done before.”