Stem cells are the primary cells that can be trained to develop into any human cell tissue eg (muscle, skin, brain, etc.) However, in embryonic stem cell research, to retrieve stem cells from embryos requires a process called therapeutic cloning where a person's DNA is put into an unfertilised human egg (effectively cloning a person), the embryo is then grown for a few days before it is destroyed, producing stem cells identical to the donor person, without any likelihood of rejection once injected.
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