Another patient has received a transplant with a genetically modified pig kidney. Tim Andrews, a 66-year-old who lives in Concord, N.H., has end-stage kidney disease and had been on dialysis for more than two years to compensate for his failing kidney function.

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 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.