Genetically modified pig hearts have been transplanted into humans in a breakthrough that could pave the way for more trials and tackle an organ shortage facing doctors.
In the past month, researchers at NYU Langone Health in the US transplanted pig hearts into two people who had recently suffered catastrophic heart failure and were left brain dead but remained on life support.
In both cases the new hearts had strong beats and were not rejected immediately by the host bodies - functioning well for the duration of the three-day experiment.
"The heart was literally banging away. It was contracting completely normally," surgeon Dr Nader Moazami said.
"We learned a tremendous amount."
The doctors hope that their research model - of testing pig organs in clinics with deceased patients - can help prepare the medical community for clinical trials and reduce the chances that living patients' immune systems will turn on new organs.
The research is being conducted against a background of organ shortage in the US, with less available for transplant than are needed by patients.
It is hoped that pig organs - if proven to be safe and effective - could alleviate the pressure on long transplant waiting lists and save lives.
"It's all about going into the first living human trials with as much data as we can possibly have and make it as safe as possible - and effective," said Dr Robert Montgomery, the director of the NYU Langone Transplant Institute.
The latest development comes a few months after a man who received a pig heart in a ground-breaking transplant died two months after surgery.
David Bennett, 57, was already terminally ill when he received a genetically modified pig heart at a hospital in Maryland.