Nuclear transfer is a laboratory procedure that creates embryos for use in stem cell research; sometimes referred to as “therapeutic cloning.” For example, nuclear transfer could be used to create a line of embryonic stem cells genetically identical to a particular patient. These embryonic stem cells could then be used to generate specialized cells that are transplanted into the patient to replace cells lost to injury or disease. When used in a medical treatment, this would ensure that the new cells would not face rejection by the patient's immune system. Nuclear transfer also gives researchers the ability to create stem cell lines that carry genetic defects that cause inherited human diseases, allowing them to study the origin of these diseases and potentially to develop new treatments.

The way nuclear transfer is done is to take an unfertilized egg cell (oocyte) from a friend, relative or clinic donor. Its nucleus (DNA) is removed and replaced with the DNA from the patient who needs the cells. Often the patient's nucleus is taken from a skin cell. Once the nucleus has been transferred, the egg is now genetically identical to the patient. This egg is abnormal for many reasons and cannot develop normally, but it can be tricked into beginning to divide (despite never having been fertilized). It undergoes cell divisions, eventually forming a blastocyst-like structure. The inner cell mass of the blastocyst is then removed to generate an embryonic stem cell line. This embryonic stem cell line is genetically identical to the patient whose skin cell was used as the source of the donor nucleus, and mature cells derived from this embryonic stem cell can then be transplanted into the patient without fear of immune rejection [Source: Life Science at University of Michigan Research].

The nuclear transfer methodology is still in its infancy. Researchers have been able to successfully perform nuclear transfer in animals, like mice, to derive embryonic stem cell lines. However, this has not yet been successfully done with human cells, though Harvard University and the University of California at San Francisco have announced research programs to achieve this goal. Ultimately, researchers believe that the knowledge gained about developmental biology via this research may make it possible for future researchers to create individualized embryonic stem cell lines without needing fertilized eggs as sources.

Reproductive cloning is the process by which nuclear transfer products are implanted in a uterus to create a pregnancy, rather than being used in laboratory dishes to generate embryonic stem cells as in therapeutic cloning. But while reproductive cloning has been achieved with a variety of animal species (such as “Dolly” the sheep), this process is highly inefficient because nuclear transfer products rarely develop normally enough to establish a viable pregnancy when they are transferred into a uterus. Only around 1% of attempts at reproductive cloning in animals give rise to viable offspring. As a result, scientists believe that reproductive cloning would be unsafe to attempt in humans, in addition to being ethically questionable. As a result, there is broad agreement among scientists and physicians that while therapeutic cloning is important, that reproductive cloning should be banned. This can be achieved by making it illegal to transfer nuclear transfer products into a woman's uterus for the purpose of establishing a pregnancy.

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