It was the fourth pig kidney transplant in the United States, and the first of three that will be done at Mass General as part of a new clinical trial sanctioned by the Food and Drug Administration. Two of the previous patients died shortly after the procedure, including one who was critically ill before the transplant.

The second person ever has received a transplant with a genetically modified pig kidney.

The surgery comes at a turning point in the quest to tell if animal-to-human transplants could help ease the shortage of donated human organs.