Xenotransplantation is the transplantation of tissue or organs from one species to another. The development of this technique has led to the breeding of animals specifically for use as human organ donors. Because humans would reject a nonhuman organ as foreign, transgenic animals (for example, pigs) are genetically altered with human DNA with the hope of suppressing any eventual rejection. One of the major risks of xenotransplantation is the risk of transplanting animal viruses along with the transfer cells or organs. Since the patient is already immunosuppressed, the patient could die from the viral infection, or the virus could be spread to the general population. But that doesn’t mean xenotransplantaion has not been successful for humans: For example, inert heart valves from pigs have been used in human heart valve-replacement operations, and scientists continue to test animals to treat human disease, such as using fetal pig cells, to treat strokes, epilepsy, and spinal cord injuries.