**1. Introduction**

According to the Organ Procurement and Transplantation Network, there are currently over 110,000 patients on the waiting list for organ transplants. Over the past 50 years, therapeutic advances and improvements in surgical techniques have increased the number of patients who could survive and benefit from organ transplantation. Unfortunately, the number of organs available through donation has not changed significantly. Thus, there is a growing disparity between organ supply and demand. Although efforts to enlarge the human donor pool have improved organ availability, even a massive expansion in organ donation would not ensure that a compatible organ would be available when and where necessary for a patient in need. Therefore, other alternatives besides expanded human donation are required.

A variety of efforts are making substantial progress in addressing the lack of organs. One area of research that is rapidly approaching clinical reality is xenotransplantation, the use of animal tissues and organs to treat patients [1]. Aside from the potential for creating an unlimited supply of organs, recent advancements in genome engineering technologies allows the genetic modification of animals to produce donor organs which are less prone to rejection for xenotransplantation in human patients.
