Several developments have fueled the renewed interest in xenotransplantation--the use of live animal cells, tissues and organs in the treatment or mitigation of human disease. The world-wide, critical shortage of human organs available for transplantation and advances in genetic engineering and in the immunology and biology of organ/tissue rejection have renewed scientists' interest in investigating xenotransplantation as a potentially promising means to treat a wide range of human disorders. This situation is highlighted by the fact that in the United States alone, 13 patients die each day waiting to receive a life-saving transplant to replace a diseased vital organ.