Definitions

Stem cells --- cells which have the potential to develop into many different cell types in the body. Can serve as a sort of repair system for the body and may theoretically divide without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.  

Embryonic stem cells—Primitive cells derived from a 5-day preimplantation embryo that has the potential to become a wide variety of specialized cell types.

Adult stem cell—An unspecific cell found in a specific tissue that can renew itself and differentiate (with certain limitations) to give rise to all the specialized cell types of the tissue from which it originated. It is important to note that scientists do not agree about whether or not adult stem cells may give rise to cell types other than those of the tissue from which they originate.   There are then two types of cloning involved with the use of either of these stem cells: therapeutic cloning and reproductive cloning.

Therapeutic cloning—The goal of therapeutic cloning is to create cells that exactly match a patient. By combining a patient's adult stem cell nucleus and an egg that has had its nucleus removed, a scientist may harvest embryonic stem cells from the resulting embryo that can be used to generate tissues that match a patient's body. This means the tissues created are unlikely to be rejected by the patient's immune system.

Reproductive cloning—The goal of reproductive cloning is to create an animal being identical to the animal that donated the adult stem cell nucleus. The embryo is then implanted in a uterus and develops into a live being. The first animal to be created by reproductive cloning was Dolly the sheep, born at the Roslin Institute in Scotland in 1996.