Therapeutic cloning involves taking an egg (or oocyte) from which the nucleus has been removed, and replacing that nucleus with DNA from the cell of another organism. The result is a blastocyst (an early stage embryo with about 100 cells) with almost identical DNA to the original organism.

The aim of carrying out this procedure is to obtain stem cells that are genetically matched to the donor organism. For example, if a person with Parkinson's disease donated their DNA, then it should be theoretically possible to generate embryonic stem cells that could be used to treat their condition without being rejected by the patient's immune system. This procedure is very controversial, and it all depends on how well each DNA cell interacts with the original organism.