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.

Over the course of the experiment, the patient showed increasing signs of organ rejection before scientists at the First Affiliated Hospital of Guangzhou Medical University in China terminated the experiment, allowing the recipient to pass away.

It's the first time a pig lung has been transplanted into a human patient, demonstrating a significant step forward, and giving scientists new problems to solve as they develop this emerging medical technique further.