Although 54 percent of adults in the United States have registered as organ donors, just one in three people die in a way that allows for organ donation. That leaves more than 100,000 people in the United States waiting for a transplant. Many will die waiting.

Because demand for organs outpaces supply and probably always will, researchers have looked to xenotransplantation — placing animal organs into human bodies — as an alternative. However, getting to the point where xenotransplantatio