Researchers want to make clones for a variety of reasons, but one of the most contentious is as a source of embryonic stem cells. These powerful stem cells can give rise to any cell or tissue type in the human body and the hope is some day they may be used to tailor medical treatments for injuries or diseases such as Parkinson’s or diabetes.

In some countries, such as Britain, their use is not controversial and is actively funded and encouraged. In the US, their use is legal but federal funding of the work is strictly limited by Congress and by President George W. Bush, who has vetoed legislation that would broaden it.

Researchers also routinely make chimeras—animals that contain the genetic material from more than one individual. These include animals that carry human genes, most commonly mice engineered with human genes that are used to study disease. (Reuters)