A man, David Bennett, said to have been dying from terminal heart failure, has become the first patient in the world to get a heart transplant from a genetically-modified pig.
David Bennett underwent the nine-hour experimental procedure at the University of Maryland Medical Center in Baltimore on Saturday.
Surgeons used a heart taken from a pig that had undergone gene-editing to make it less likely that his body's immune system would reject the organ.
The 57-year-old patient is breathing on his own while still connected to a machine that helps his new heart pump blood around his body.
Experts say it is too soon to know if his body will fully accept the organ and that the next few weeks will be critical as he is weaned off the machine.
But, if successful, it would mark a medical breakthrough and could save thousands of lives in the US alone each year.
Doctors called the procedure a 'watershed event.'
Mr Bennett, a labourer, knew there was no guarantee the risky operation would work but was too sick to qualify for a human organ.
A day before his pioneering surgery, Mr Bennett said it was 'either die or do this transplant', he said, "I want to live. I know it's a shot in the dark, but it's my last choice."
There is a huge shortage of human organs donated for transplant in the US and the UK, driving scientists to try to figure out how to use animal organs instead.
Nearly 120,000 Americans are in need of healthy organs and, on average, 20 people die each day waiting for one to become available.
Last year, there were just over 3,800 heart transplants in the US, a record number, according to the United Network for Organ Sharing (UNOS), which oversees the nation’s transplant system.
But prior attempts at such transplants – or xenotransplantation – have failed, largely because patients' bodies rapidly rejected the animal organ. Notably, in 1984, 'Baby Fae' — who was born with a rare heart condition — lived 21 days with a baboon heart.
Bennett, who has been relatively healthy most of his life, began having severe chest pains in October, his son said.
He went into the University of Maryland Medical Center with severe fatigue and shortness of breath.
"He couldn't climb three steps," said David, a physical therapist who understood the seriousness of his father's condition.
Griffith told the New York Times, "It creates the pulse, it creates the pressure, it is his heart.
"It's working and it looks normal. We are thrilled, but we don't know what tomorrow will bring us. This has never been done before."
Griffith said the patient's condition - heart failure and an irregular heartbeat - made him ineligible for a human heart transplant or a heart pump.
Bennett also failed to qualify for the waitlist for human heart transplants because he had not followed doctors' orders, missing medical appointments and discontinuing prescribed medications.
