Animal organs for human transplantation: how close are we?

Solid organ transplantation has been, by most measures, a phenomenal success. Nonetheless, the field is plagued by extreme shortages of available organs from a very limited number of donors. One potential solution to this organ availability crisis is the use of animals as organ donors for humans (xenotransplantation). Though the concept remains theoretical, significant advances are being made in the field of genetics and in our understanding of the immunological barriers to xenotransplantation. With these advances also comes increased knowledge about the potential risks of xenotransplants, especially disease transmission. The eventual clinical application of animal-to-human transplants will require a careful, balanced appraisal of these issues.