Large numbers of transgenic animals could produce vast quantities of needed drugs and other useful substances more efficiently and at much lower cost than is possible with bioengineering methods. As of mid-1998, most genetically engineered proteins were being manufactured in bioreactors, large steel vessels in which billions of genetically modified microorganisms produce proteins that are then extracted and purified. However, human cloning would probably be more difficult than sheep or cattle cloning, because the cells of human embryos start producing proteins at a relatively early stage. Thus, there would not be as much time for the egg cytoplasm to reprogram a transplanted nucleus. However, the successful 1998 cloning of mice, which also start producing proteins at an early embryonic stage, strongly indicated that this problem can be overcome in humans.
Geneticists foresee a number of practical applications for the cloning of humans.
Infertile couples who do not wish to adopt, for instance, could use cloning to have children who are biologically related to them. Cloning could also be used to produce offspring free of certain diseases. If a woman were to carry a gene for one of these disorders, she could conceive a healthy child by having the nucleus of one of her body cells inserted into an enucleated egg cell from a woman who does not have anything wrong with her mitochondrial genes. The resulting embryo could then be implanted into the woman who donated the nucleus, and she would carry the baby to term. The mass production of animals with genetically modified organs that could be safely transplanted into humans.