Researchers have uncovered and then overcome an obstacle that has led to the failure of pioneering efforts in xenotransplantation, in which an animal kidney is transplanted into a human.

More than 800,000 Americans have late-stage kidney disease, yet only 3% receive a transplant each year, according to the U.S. Centers for Disease Control and Prevention. To boost the supply of available organs, experts are exploring the use of genetically modified pig kidneys.

The genetic changes are meant to keep the human immune system from recognizing the animal organ as foreign and attacking it to cause rejection. However, recipients’ immune reactions can still lead to organ damage and failure after the surgery.