Some researchers have suggested the possibility of mtDNA being responsible for aging. In a few years, it may be found that Dolly and the other mammalian clones age prematurely. There are even more problems with cloning humans. The development of the human brain depends heavily on the physical stimulants it receives early in life. Therefore, cloning a great musician could result in a tone-deaf scientist, if the same musical sounds are not heard before the brain cells lose their plasticity.
Current legislation prevents the cloning of humans but is unspecific.
An agreement between 19 European nations prohibits the cloning of humans. Several U.S. Federal and State Legislature bills are currently under discussion for the limiting of human cloning research. The cloned human embryo made in the U.S. was destroyed at 12 days, two days before the embryo would have naturally implanted into the uterine wall, to avoid legal repercussions. The ethical considerations with human cloning appear to address the morality of manufacturing a human being. Twins are technically clones but they have no knowledge of what they are to be like as adults, nor do they have presupposed expectations to becoming someone else.