An important new source of embryonic stem cells is through use of a technique called somatic cell nuclear transfer, also referred to a therapeutic cloning. In this technique, nuclei from the cells of living patients with specific diseases are isolated and used for the generation of embryonic stem cells. This is achieved by placing one such nucleus into a donated unfertilized egg from which the genetic material has been removed and then stimulating the egg to divide to the stage when stem cells can be derived in culture. These cells can then be induced to develop in the laboratory into specialized cells such as nerve cells that are affected by the disease in question. In the future, by watching what happens to these cells, and how they differ from normal cells over time, scientists can learn about the pathogenesis of the disease and about interventions that may be used to treat it. In addition, embryonic stem cells derived by this technique ultimately may be useful for cell-based replacement therapy for the original patients from whom they were derived. This is because they are genetically identical to the patient and are not rejected by the immune system. While an important step toward realizing the promise of stem cell research, this approach raises new ethical and regulatory issues that must be addressed carefully.