Transplantation of two genetically modified pig hearts.
A team of scientists in the United States who transplanted genetically modified pig hearts earlier this year have managed to carry out two such interventions.
In January David Bennett, 57, became the first person in the world to receive a genetically modified pig heart.
Before this surgical intervention, pig kidneys were transplanted, but the patients suffered from brain attacks.
The organs were provided by Revivicor, which uses genetic engineering to remove certain genes in order to reduce the risk of organ transplant failure and make the organ more acceptable to the human immune system.
After January, two more pig heart transplants were published, one in June and the other in July.