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Scientific and Medical of Aspects: Human Reproductive Cloning
mammals, including humans, and at the present time, we do not know whether attempts at human cloning would reveal fewer, more, or different abnormalities.
3-1. In general, the efficiency of reproductive cloning in animals remains extremely low despite several years of experimentation.
3-2. Animal cloning results in a wide variety of abnormalities, including greater than normal size (both during gestation and after birth), greater early- and late-gestation fetal morbidity and mortality, greater postnatal mortality, and various developmental defects in the immune, cardiovascular, and possibly nervous systems. (Subtle behavioral and mental defects might be undetectable in animal models.) In addition to the risks inherent in the overproduction of oocytes from egg donors, increased maternal morbidity and mortality are to be expected.
3-3. The most likely reasons for the abnormalities are failures in reprogramming in the adult nucleus used for reproductive cloning, so that it fails to turn on all the appropriate embryo-specific genes at the right times, and errors in imprinting.
3-4. Before human reproductive cloning is feasible, a great deal more research is necessary, including studies of cloning in nonhuman primates. Research focused on gaining an understanding of all aspects of reprogramming and imprinting, determining which steps in the reproductive cloning technique contribute to the overall low efficiency, and determining how these problems can be overcome would be most useful.
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