“If you put all the organs on a spectrum, the lung will always be the hardest to do,” said Dr. Leonardo Riella of Massachusetts General Hospital, who performed the first successful transplant of a kidney from a gene-edited pig into a living human.

A genetically modified pig lung transplanted into a brain-dead human patient functioned for nine days in a new achievement that reveals both the promise and significant challenges of xenotransplantation.

In recent years, scientists have transplanted kidneys, livers, and hearts from gene-edited pigs into a human body. But this new research appears to show the first recorded instance of a gene-edited lung transplant.