Dave Bennett, the Maryland man who received the first heart transplant from a genetically modified pig last week, is doing well, according to his doctors, who released an update late Wednesday.

“The new heart is still a rock star,” said Dr. Bartley Griffith of the University of Maryland Medical Center, who led the transplant team. “It appears to be content in its new home… It has far surpassed our expectations.”

Bennett, 57, has been taken off the machine that has kept blood circulating through his body for more than 45 days, including several days after surgery. He is breathing on his own and speaking in hushed tones.

Griffith intended to keep Bennett hooked up to the heart-lung machine for another week or so, comparing it to “training wheels” while the pig heart adjusted to its new surroundings. “But the heart was rocking and rolling, and he was so stable that we decided to take it out,” he explained.

Bennett’s son, David Bennett Jr., said in a video recorded by the University of Maryland Medicine that his father can’t wait to get out of the hospital and is grateful for the groundbreaking surgery that may give him that chance.

“My father is a fighter,” David explained. “He was chosen for this task. He made the decision to do so.”

Researchers, including Muhammad Mohiuddin, a physician-scientist at the University of Maryland School of Medicine, have been working for decades to develop pigs whose organs can be used to address the global organ shortage. Currently, over 100,000 Americans are on the waiting list for an organ transplant, with approximately 6,000 dying each year.

More, like Bennett, aren’t even on the list. Bennett was deemed unsuitable for a heart transplant because he had a history of missing medical appointments and failing to fill prescriptions. Years of heart transplant surgery have demonstrated that people who are not good at following doctors’ orders do not fare well with a donor heart. Bennett was also deemed unsuitable for an implantable device due to uncontrolled arrhythmia, also known as an irregular heartbeat.

Revivicor, a Virginia-based company, raised the 240-pound, one-year-old pig whose heart now beats inside Bennett’s chest.

Ten of the pig’s 100,000 genes had been altered to make its heart more human-compatible. The modifications reduced the likelihood that Bennett’s body would reject an organ from another species, prevented his blood from clotting as it passed through the heart, and prevented the pig from growing too large or its organ from growing after the transplant.

Years of transplanting pig hearts into baboons helped scientists identify the critical genes, as well as medications to prevent immune rejection and methods to keep the heart as healthy as possible as it transitioned from pig to person.

Some people have ethical and moral issues with the procedure, which resulted in the pig’s death. Bennett did not, according to his son, despite the fact that he was scared before the surgery and hoped to someday be able to receive a human heart transplant.

Bennett underwent heart surgery in 2013 to implant a pig valve in his heart. That procedure, which has been performed for decades, is not considered an organ transplant because it does not involve a full organ and because all pig cells are removed before implanting, allowing patients to avoid immunosuppressive medication.

When Bennett’s body did not reject the heart on the operating table or soon afterward, a condition known as hyper-acute rejection, doctors at the University of Maryland Medical Center breathed a sigh of relief. It had previously been a major impediment to animal-to-human transplants.

They are still concerned about Bennett’s infection risk and the possibility that his body will reject the heart, both of which are risks associated with any organ transplant. Mohiuddin said in a video that he was moved when Bennett thanked him. A successful pig-to-human heart transplant “was fantastic,” according to Mohiuddin, but the main goal was to save Bennett’s life.

Mohiuddin expressed gratitude, saying, “meant he was aware of what had occurred. It meant he realized what he had agreed to and what had happened to him, which was a very emotional feeling.”