There are more than 120,000 people in the U.S. waiting for an organ transplant and not enough donors. The dire shortage has led some researchers to consider an unusual solution: They are breeding genetically modified pigs whose organs could be compatible for human transplant. Researchers have been trying for decades to make animal-to-human transplants work, a process known as xenotransplantation. Pigs are a particularly promising source of organs. They produce big litters. Organs such as the kidney and liver are similar in size to those of humans. “Nobody has come up with a better animal,” says Joseph Tector, a professor of surgery who runs the xenotransplantation program at the University of Alabama at Birmingham.