Wolf says that there is a need for drastic measures. "The situation is very pressing," he says. "For instance, only about half of the patients who are on the active waiting list for a heart can receive a transplant. Pigs have a number of advantages as donors because the size and function of the organs is relatively fit for humans, genetic engineering is well established in pigs, and the use of pigs is more ethically accepted than non-human primates."