them, would be genetically identical with the patient’s own and not be rejected by the immune system. However, as previously described, the availability of sufficient adult stem cells and their potential to give rise to a full range of cell and tissue types are uncertain. Moreover, in the case of a disorder that has a genetic origin, a patient’s own adult stem cells would carry the same defect and would have to be grown and genetically modified before they could be used for therapeutic transplantation.

The application of somatic cell nuclear transfer or nuclear transplantation offers an alternative route to obtaining stem cells that could be used for transplantation therapies with a minimal risk of transplant rejection. This procedure—sometimes called therapeutic cloning, research cloning, or nonreproductive cloning, and referred to here as nuclear transplantation to produce stem cells—would be used to generate pluripotent ES cells that are genetically identical with the cells of a transplant recipient [50]. Thus, like adult stem cells, such ES cells should ameliorate the rejection seen with unmatched transplants.

Two types of adult stem cells—stem cells in the blood forming bone marrow and skin stem cells—are the only two stem cell therapies currently in use. But, as noted in the National Academies’ report entitled Stem Cells and the Future of Regenerative Medicine, many questions remain before the potential of other adult stem cells can be accurately assessed [11]. Few studies on adult stem cells have sufficiently defined the stem cell’s potential by starting from a single, isolated cell, or defined the necessary cellular environment for correct differentiation or the factors controlling the efficiency with which the cells repopulate an organ. There is a need to show that the cells derived from introduced adult stem cells are contributing directly to tissue function, and to improve the ability to maintain adult stem cells in culture without the cells differentiating. Finally, most of the studies that have garnered so much attention have used mouse rather than human adult stem cells.

ES cells are not without their own potential problems as a source of cells for transplantation. The growth of human ES cells in culture requires a “feeder” layer of mouse cells that may contain viruses, and when allowed to differentiate the ES cells can form a mixture of cell types at once. Human ES cells can form benign tumors when introduced into mice [20], although this potential seems to disappear if the cells are allowed to differentiate before introduction into a recipient [51]. Studies with mouse ES cells have shown promise for treating diabetes [30], Parkinson’s disease [52], and spinal cord injury [53].

The ES cells made with nuclear transplantation would have the advantage over adult stem cells of being able to provide virtually all cell types and of being able to be maintained in culture for long periods of time. Current knowledge is, however, uncertain, and research on both