Every year, around 8,000 people die waiting for an organ transplant. There just doesn't seem to be enough human organs available for those who need them. Biotech company United Therapeutics has been investigating ways to grow pig organs to be used in humans, and in the four years its researchers have been working on it, the company has become the largest commercial backer of xenotransplantation (transplants between species) and has found some initial success.

In the past, doctors tried to put pig hearts and livers in people, however these attempts did not go well—patients died, and their doctors were often charged with murder. That's because cross-species transplantation sets off a powerful response in the recipient's immune system, and even potent immunosuppressant drugs (taken by all transplant recipients, no matter the species of their donor) can't completely stop it.