Besides making embryonic stem cells from unwanted embryos, many scientists hope to make embryonic stem cells that are genetically identical to an existing human, a process called somatic cell nuclear transfer (SCNT) or therapeutic cloning. At some point, scientists might be able to use such genetically matched cells to help replace or supplement a patient's failing organ, like the cells of the pancreas that secrete insulin. Closer to hand, scientists could work to understand how genes and environment collude to cause disease. (For example, custom-made stem cells from a person known to have, say, a degenerative disease, could be tested under different conditions or with different drugs to find which keeps the cells most healthy.)

Suppose researchers wanted to make embryonic stem cells genetically identical to Monya Baker, editor of Nature Reports Stem Cells (or any other individual, male or female). They would first collect some cells from Monya, most easily from skin or blood, and a large supply of unfertilized human eggs (from Monya or anyone else). Researchers would replace the nucleus of one of the eggs with the nucleus from one of Monya's cells. Next, they'd prompt the egg to divide again and again for a few days until it became a blastocyst. Then, they'd make embryonic stem cells just as they are made from unused embryos in fertility clinics.

Credit: Jessica Kolman

Right now, that attempt would almost certainly fail. In fact, in one of the biggest scientific scandals of this century, scientist Woo-suk Hwang faked data, claiming to have achieved this anticipated breakthrough when in fact he had not. As of April 2007, no one has been able to perform the technique described above, which is called SCNT, nuclear transfer or therapeutic cloning. It has worked, however, for sheep, mice, cows, monkeys and other mammals, and there is no overwhelming theoretical reason why it should not work for humans; it just seems to be particularly challenging technically.