US surgeons who transplanted a genetically modified pig kidney into a brain-dead patient announced Thursday they had ended their experiment after a record-breaking 61 days.

The latest experimental procedure is part of a growing field of research aimed at advancing cross-species transplants, mainly testing the technique on bodies that have been donated for science.

There are more than 103,000 people waiting for organ transplants in the United States, 88,000 of whom need kidneys.

"We have learned a great deal throughout these past two months of close observation and analysis, and there is great reason to be hopeful for the future," said Robert Montgomery, director of the New York University Langone Transplant Institute, who led the surgery in July.

It was the the fifth so-called xenotransplant performed by Montgomery, who also carried out the world's first genetically modified pig kidney transplant in September 2021.

Tissue collected during the study indicated a mild rejection process had begun, requiring intensification of immunosuppression medication.

By "knocking out" the gene responsible for a biomolecule called alpha-gal — a prime target for roving human antibodies — the NYU Langone team were able to stop immediate rejection.

The donor pig in this experiment came from a herd cultivated by Virginia-based biotech company Revivicor.

The herd has also been approved by the Food and Drug Administration as a source of meat for people with hypersensitivity to the alpha-gal molecule, an allergy caused by some tick bites.

These pigs are bred, not cloned, meaning the process can be more easily scaled.

Early xenotransplantation research focused on harvesting organs from primates — for example, a baboon heart was transplanted into a newborn known as "Baby Fae" in 1984, but she survived only 20 days.

Current efforts focus on pigs, which are thought to be ideal donors for humans because of their organ size, their rapid growth and large litters, and the fact they are already raised as a food source.