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“I really wanted to contribute to the science of it,� Bill Stewart, an athletic trainer from Dover, New Hampshire, told The Associated Press.

That’s not the only milestone the Mass General team is marking: A pig kidney has kept another New Hampshire man, for a record seven months and counting. Until now, the longest that a gene-edited pig organ transplant was known to last .

Based on lessons from the New Hampshire men and a handful of other one-off attempts, the Food and Drug Administration approved pig producer eGenesis to begin a rigorous study of kidney xenotransplants.

“Right now we have a bottleneck� in finding enough human organs, said Mass General kidney specialist Dr. Leonardo Riella, who will help lead the new clinical trial.

More than 100,000 people are on the U.S. transplant list, most who need a kidney, and thousands die waiting. As an alternative, scientists are so their organs are more humanlike, less likely to be immediately attacked and destroyed by people’s immune system.

Initial experiments, two hearts and two kidneys, were short-lived and included very ill patients. also recently announced a kidney xenotransplant but released little information. Then an Alabama woman whose pig kidney lasted 130 days before rejection prompted its removal, sending her back to dialysis, helped researchers shift to not-as-sick patients.

In New Hampshire, high blood pressure caused Stewart's kidneys to fail but he had no other health problems. It can take up to seven years for people with his blood type to find a matching kidney from a deceased donor, and some would-be living donors didn't qualify. After two years in dialysis, he heard about Mass General’s most recent xenotransplant recipient – Andrews – and applied to be the next candidate.

“I’ve always been a little bit of a science nerd,� Stewart said. Conscious of how new these experiments are, he sought out Andrews for advice and ultimately decided, “worst case scenario, they can always take it out.�

Thrilled to no longer have his time and energy sapped by dialysis, Stewart said he's easing back into desk duties at work and visited his old dialysis clinic to “let everyone know I’m doing all right and maybe kind of give some people some hope.�

Riella, the kidney specialist, said Stewart had his anti-rejection drugs adjusted to counter an early concern and that Andrews has needed similar adjustments. He said it’s far too early to predict how long pig kidneys might be able to last — but it would be useful even if initially they can buy people time off dialysis until they get a matching human organ.

“A year, hopefully longer than that – that’s already a huge advantage,� he said.

The new eGenesis trial will provide gene-edited pig kidney transplants to 30 people age 50 or older who are on dialysis and the transplant list. Another developer of gene-edited pig organs, United Therapeutics, is about to start enrolling people in a similar FDA-approved study.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.