Cloning describes the processes used to create an exact genetic replica of another cell, tissue or organism. The copied material, which has the same genetic makeup as the original, is referred to as a clone. The most famous clone was a Scottish sheep named Dolly.

There are three different types of cloning:

• Gene cloning, which creates copies of genes or segments of DNA
• Reproductive cloning, which creates copies of whole animals
• Therapeutic cloning, which creates embryonic stem cells. Researchers hope to use these cells to grow healthy tissue to replace injured or diseased tissues in the human body.

The term cloning is used by scientists to describe many different processes that involve making duplicates of biological material. In most cases, isolated genes or cells are duplicated for scientific study, and no new animal results. The experiment that led to the cloning of Dolly the sheep in 1997 was different: It used a cloning technique called somatic cell nuclear transfer and resulted in an animal that was a genetic twin -- although delayed in time -- of an adult sheep. This technique can also be used to produce an embryo from which cells called embryonic stem (ES) cells could be extracted to use in research into potential therapies for a wide variety of diseases.

Thus, in the past five years, much of the scientific and ethical debate about somatic cell nuclear transfer has focused on its two potential applications: 1) for reproductive purposes, i.e., to produce a child, or 2) for producing a source of ES cells for research.