In an operation that could prove to be a milestone for people in need of organs, surgeons in the US transplanted a genetically-modified pig’s kidney to the man.

The procedure was performed at the Massachusetts General Hospital in Boston and took four hours. The patient, Richard Slayman, is in good health and will likely to be discharged soon.

Experts are keenly interested in long-term results of the groundbreaking animal-to-human transplant, said Dr. Jim Kim, director of kidney and pancreas transplantation with USC Transplant Institute in Los Angeles.

Slayman had received a transplant of a human kidney at the same hospital in 2018 after seven years on dialysis, but the organ failed after five years and he had resumed dialysis treatments.

The kidney was provided by eGenesis of Cambridge, Massachusetts, from a pig that had been genetically edited to remove genes harmful to a human recipient and add certain human genes to improve compatibility. The company also inactivated viruses inherent to pigs that have the potential to infect humans.

Kidneys from similarly edited pigs raised by eGenesis had successfully been transplanted into monkeys that were kept alive for an average of 176 days, and in one case for more than two years, researchers reported in the Nature journal.

Drugs used to help prevent rejection of the pig organ by the patient’s immune system included an experimental antibody called tegoprubart, developed by Eledon Pharmaceuticals (ELDN.O.

The surgery marks progress in xenotransplantation – the transplanting of organs or tissues from one species to another - said Dr. Robert Montgomery, director of the NYU Langone Transplant Institute, who was not involved in the case.

The field “is marching closer to becoming an alternative source of organs for the many hundreds of thousands suffering from kidney failure,” he said in an email.

According to the United Network for Organ Sharing, more than 100,000 people in the U.S. await an organ for transplant, with kidneys in the greatest demand.

NYU surgeons had previously transplanted pig kidneys into brain-dead people.

Montgomery said transplant centers are taking different approaches in terms of gene edits and medications, adding that “another big step will be when the FDA authorizes clinical trials so we may better understand what will work best for patients on our waiting lists.”

A University of Maryland team in January 2022 transplanted a genetically modified pig heart into a 57-year-old man with terminal heart disease, but he died two months later.