The production of α1,3-galactosyltransferase gene-knockout (GT-KO) pigs has overcome the barrier of preformed anti-Galα1,3Gal (Gal) antibodies that has inhibited progress in pig-to-primate organ xenotransplantation for many years. Survival of GT-KO pig organs in nonhuman primates is currently limited by the development of a thrombotic microangiopathy that results in increasing ischemic injury of the transplanted organ over weeks or months. Potential causative factors include vascular endothelial activation from preformed anti-nonGal antibodies or cells of the innate immune system that recognize nonGal pig antigens directly, and coagulation dysregulation associated with molecular incompatibilities between pig and primate. Carefully isolated pancreatic islets from wild-type (genetically unmodified) adult pigs express minimal Gal epitopes, allowing survival sometimes for weeks or months after transplantation into nonhuman primates receiving immunosuppression directed only at T-cell function. However, there is a considerable immediate loss of islets, probably related to activation of coagulation and complement cascades. Further genetic manipulation of organ-source pigs is therefore required to overcome these problems. GT-KO pigs expressing a human complement-regulatory protein, e.g. decay-accelerating factor, and/or an 'anti-coagulant' gene, e.g. human tissue factor pathway inhibitor, might prevent the change in vascular endothelium from an anti-coagulant to a procoagulant phenotype, and protect the islets from early loss. © 2007 European Society for Organ Transplantation.