theoretically increase the chances of matching the histocompatibility antigens of a transplant recipient. It has also been suggested that ESCs could be made less reactive by using genetic engineering to eliminate or introduce the presence of surface antigens on them (Odorico, 2001). An exact genetic match between a transplant recipient and tissue generated from ESCs could also, in theory, be achieved by using somatic cell nuclear transfer to create histocompatible ESCs (Figure 4). Cells created with this technique would overcome the problem of immune rejection. However, it might to not be appropriate to transplant such cells in a person with a genetically based disease, since the cells would carry the same genetic information. In any case, an understanding of how to prevent rejection of transplanted cells is fundamental to their becoming useful for regenerative medicine and represents one of the greatest challenges for research in this field.