**5. Current status and challenges for xenotransplantation**

A variety of academic, clinical and industrial institutions have made substantial progress in recent years in the understanding of the molecular mechanisms of the xenorejection response and the genetic modification of pigs to overcome these mechanisms [18]. Professional organizations are working with the FDA to develop guidelines for clinical use [19], with several groups indicating their intention to initiate clinical trials in the near term with porcine organs [20].

For xenotransplantation to become a viable routine human therapeutic option, there are a number of challenges that still need to be overcome. These challenges fall into two broad categories; the biology of the xenorejection responses and the engineering technologies needed to restructure the porcine genome to overcome these responses.
