With donor organs unavailable for most of the 80,000 people awaiting transplants in the United States, scientists are working to overcome the two biggest hurdles to xenotransplantation—immune rejection and infection. A Massachusetts company, Immerge BioTherapeutics, in collaboration with researchers around the country, has eliminated a gene in a cloned “knockout” pig that produces a key enzyme in the rejection process. The company has also identified swine that do not produce porcine endogenous retrovirus, which has been found to infect human cells in vitro.

“The waiting list for transplants continues to grow,” Julia L. Greenstein, Ph.D., president and CEO of Immerge, said in a January talk sponsored by the Interdepartmental Program in Vascular Biology and Transplantation. “For the most part the donor list has remained incredibly static. We need to be able to do something else to address the patients who are on the waiting list and are never going to get organ transplants.”