The first attempts to turn mature cells back into pluripotent stem cells involved a process called cloning. In the cloning process, an egg is fertilized in the lab, and right after fertilization, the DNA is removed from the egg. The empty egg is then injected with DNA from another mature cell, such as a skin or blood cell. Even though the DNA is from a mature cell, the environment of the egg will basically reprogram the genetic material from the mature cell, so that it can create an embryo. If the egg keeps developing, it will develop into a clone of the person or animal from which the mature cell was taken. Human cloning is illegal, but in the early 1960's, English researchers successfully cloned frogs. Frogs have relatively very large eggs, so they are easy to work with. The researchers took a fertilized egg from a frog, removed the DNA, and injected the egg with genetic material from an intestinal cell of another frog. After about 40 days, the egg matured and developed into a tadpole. The tadpole was genetically identical to the frog from which the intestinal cell was taken.

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