Therapeutic cloning is a technique that would be used to produce cloned embryos, but only to create stem cells that can in turn be used to repair damaged or defective tissue in the parent of the cloned cells. Such stem cells could theoretically be used to grow replacement livers or hearts (or any of a variety of organs), for transplant without fear of rejection. They might be used to create healthy nerve cells for people with Alzheimer's or Parkinson's disease. Skin cells could be derived from cloned stem cells for burn victims.

The person who needed the healthy stem cells would provide a non-egg, non-sperm cell from which the DNA would be removed. That DNA, containing two copies of each human chromosome, would be inserted into a donor egg that has had its own nucleus and DNA removed. The egg with the introduced DNA would act like it had just been fertilized and begin to divide, forming an embryo. Stem cells from that embryo would be removed and cultured to provide the needed healthy tissue.

Another form of therapeutic cloning would be used for infertility treatment or to assist couples who know that one parent carries a gene for a potentially lethal disease.

A final and very speculative version of therapeutic cloning might be to clone a deceased individual from some preserved tissue containing intact DNA. An unsuccessful attempt at this has already been reported. One set of grieving parents paid the corporation Clonaid half a million dollars to try to clone their deceased infant daughter from her preserved skin cells.

Although human stem cells hold a lot of promise for regeneration of damaged organs and tissues throughout the body, in many cases scientist are a very long way from harnessing the therapeutic potential of these cells. For example, it easy to say that stem cells may be able to replace damaged neurons in the brain or spinal cord. However, actually getting the stem cells to turn into neurons, implant them in the brain, and then have them make the right connections within the nervous system is something that is not technically feasible at the present time, and is not likely to be for many years.

In other cases, the therapeutic potential of human stem cells will likely be realized in much more immediate fashion. Examples of such applications are using human stem cells to repair damaged liver or provide new bone marrow for patients whose own marrow has been irreversibly damaged by chemotherapy. In short, stem cells hold a lot of promise, but like any therapeutic strategy, a great deal of research will be needed before their potential therapeutic value can be fully evaluated and realized.