1.14. A clear distinction must be drawn between reproductive cloning aimed at the birth of identical individuals, which in humans has never been performed, and nonreproductive cloning, limited to the in vitro phase.
1.15. In considering human implications, we must again distinguish between cloning by embryo splitting and cloning by nuclear replacement (see 1.1). We must also distinguish between nuclear replacement as a means of cloning and nuclear replacement as a therapeutic measure, for example, to avoid the very serious consequences of mitochondrial disease. The latter situation, which would require an enucleated donor egg containing normal mitochondria, as it need not involve the production of genetically identical individuals, will not be considered further here although we appreciate that it will raise ethical problems of its own.
1.16. Embryo splitting in the human is the event that gives rise to monozygotic (one-egg) twins and higher multiples. It has been discussed in the context of assisted reproduction, as a means of increasing the success rate of IVF, but there is no evidence that it has ever been used for this purpose, nor that it would be effective if it were so used, because of the pattern of early development of the human embryo.
1.17. Monozygotic twins show us that genetically identical individuals are far from identical: they may differ from one another not only physically but also psychologically, and in terms of personality. Individuals cloned by nuclear transfer from an adult cell would of course be even more different from their donor, since they would have different mitochon-drial populations, they would be different in age, and they would have had a different environment both before and after birth and a different upbringing. We are not just our genes.
1.18. There is no ethical objection to genetically identical human beings per se existing, since monozygotic twins are not discriminated against. However, the use of embryo splitting, or the use of human embryo cells as nuclear donors, deliberately to produce genetically identical human beings raises serious ethical issues, concerned with human responsibility and instrumentalization of human beings.
1.19. However, research involving human nuclear transfer could have important therapeutic implications, for example, the development of appropriate stem cell cultures for repairing human organs. It could also provide insights into how to induce regeneration of damaged human tissues. If such research resulted in embryonic development, the serious and controversial ethical issues concerning human embryo research would of course arise. Any attempt to develop methods of human reproductive cloning would require a large amount of human experimentation.
1.20. If adult cells were to be used as nuclear donors, we are still ignorant of the possible risks: whether the cloned individuals would have a shorter life span, a greater susceptibility to cancer, whether they would be fertile, and if so, whether they or their offspring would suffer from an abnormal rate of genetic abnormalities. Furthermore, the procedure would be immensely costly: each attempt would require several eggs and an available uterus, and many attempts would be unsuccessful. The issues of human responsibility and instru-mentalization of human beings are even more ethically acute in this context.
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