How is stem cell research related to cloning?

Most embryonic stem cell research is conducted on embryos donated from fertility clinics. However, in some cases, research on embryonic stem cells from other species, such as mice, is conducted on cloned embryos -- those produced from somatic cell nuclear transfer, or SCNT, commonly called therapeutic cloning.

No human cases of SCNT have ever been confirmed. But as with embryonic stem cell research, scientists hope SCNT will lead to the production of organs and tissues that are genetically identical to the person getting the transplant. That would reduce or even eliminate the potential for immune rejection.

In the procedure, the nucleus is removed from a somatic cell -- any cell in the body that isn't a sperm or egg cell. Separately, the nucleus of an egg is also removed. Then the nucleus of the somatic cell is inserted into the egg cell, thus leading to the equivalent of a fertilized egg.

Scientists extract the stem cells from the blastocyst, thereby destroying the cloned embryo. The stem cells would then be grown in a lab and induced to differentiate into different types of human cells, such as blood cells, neurons or pancreatic islet cells. These cells would be suitable for therapies conducted on the original somatic cell donor, since their genetic makeup is identical.

But don't confuse the process termed therapeutic cloning with reproductive cloning, which is banned by many states and countries. In reproductive cloning, the fertilized egg that results from SCNT would be implanted into a woman's uterus to grow into a cloned human child. However, no cases of human reproductive cloning have ever been confirmed, and the idea is frowned upon by the scientific community and the vast majority of the general public.