For the second time, Massachusetts General Hospital has transplanted a genetically edited pig kidney into a human.

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.

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.