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For Tim Andrews, a kidney transplant from a genetically modified pig offered something increasingly precious to patients with failing kidneys.
The 66 year old American became the recipient of a pig kidney that functioned inside his body for 271 days, marking the longest recorded period for a transplanted pig kidney to operate in a patient, according to doctors at Mass General Brigham.
The breakthrough is being viewed as an important step in the development of xenotransplantation, a field that seeks to use animal organs to address the global shortage of organs available for human transplantation.
Andrews had end stage kidney disease caused by type 2 diabetes when he underwent the experimental procedure on January 25, 2025. Doctors reported that the pig kidney began working immediately, allowing him to spend months without relying on dialysis.
For Andrews, the possibility of escaping dialysis represented far more than a medical experiment.
He had been told that obtaining a human kidney could take as long as seven years, while the average life expectancy for someone dependent on dialysis is around five years. He described the transplant as giving him hope and the possibility of living without being tied to a dialysis machine.
The success of the procedure depended heavily on advances in genetic engineering. Researchers used Yucatan miniature pigs whose organs had undergone 69 genetic modifications designed to make them more compatible with the human body.
Without such modifications, the human immune system would recognize a pig organ as foreign and rapidly attack it. Scientists therefore removed certain pig characteristics that trigger immune responses, introduced human genetic material to help disguise the organ and altered pig DNA to reduce the risk of infections from viruses carried within the animal genome.
The transplanted kidney eventually developed complications. Doctors observed early signs of immune rejection, which were controlled by adjusting Andrews' medication. After approximately six months, however, blood vessels in the organ began deteriorating, inflammation developed and kidney function eventually declined.
The organ was removed after 271 days in October 2025. Andrews subsequently spent 82 days on dialysis before receiving a kidney from a human donor in January 2026. Doctors say that kidney is functioning well.
Dr Leonardo Riella of Mass General Brigham said the shortage of transplantable organs has created a crisis and argued that xenotransplantation could eventually provide an alternative for patients without living donors.
The latest case does not eliminate the challenges surrounding animal to human transplantation, but it demonstrates how rapidly the field is progressing.
For thousands of patients waiting for a kidney, the possibility of using genetically modified animal organs could one day transform transplantation from a race against time into a more accessible medical option.
