In the fall of 2003, researchers at Advanced Cell Technology in Worcester, MA, thought they were on the verge of a breakthrough. They had generated cloned human embryos and were coaxing those embryos to live long enough to harvest their stem cells -- the crucial, final step in generating stem cells that are perfectly matched to an individual. But in February 2004, the researchers' hopes were dashed. South Korean stem cell scientist Woo Suk Hwang and colleagues announced in the journal Science that they had achieved this feat. ACT rapidly lost its funding and supply of eggs because investors and donors assumed this important stem cell objective had already been achieved.

Now two years -- and a scientific scandal -- later, ACT scientists are devastated all over again. In December 2005, investigations revealed that Hwang faked his human cloning research. "The Koreans were so far ahead of us, we were perceived as a failure, when in fact we were the furthest ahead," says Robert Lanza, vice president of medical and scientific development at ACT. "The Korean episode is a blow for stem cell research and for the patients who may die in the future because this research has been held up," he says. ACT plans to start the program up again, as soon as it can find a new supply of eggs.

Therapeutic cloning, also called somatic cell nuclear transfer, involves inserting the nucleus of a differentiated cell, such as a skin or nerve cell, into an unfertilized egg whose nucleus has been removed. The cell then develops into an early embryo, from which stem cells can be extracted. Stem cell lines currently used in research were derived from naturally fertilized human embryos.

Therapeutic cloning has the potential to create patient-specific stem cells because scientists can generate embryos using differentiated cells from an individual, such as a skin cell. Because the cells are genetically matched to a patient, they could one day be used in transplant therapies for diseases such as Parkinson's and diabetes without the risk of immune rejection. Patient-derived stem cells could also be used to create disease-specific cell lines, which scientists can use as a human model to study disease.

While scientists have been able to generate stem cells from cloned embryos in mice, replicating that feat in humans has been a daunting task. The scientific community thought that hurdle had been overcome when Hwang first published his Science paper in 2004. Hwang elaborated on that technique in another paper in Science in June 2005, claiming to have generated 11 patient-specific stem cell lines using very few eggs. Because human eggs are difficult to obtain in great supply, an efficient cloning technology is crucial for the clinical success of stem cell therapy.

However, scientists now know that Hwang had faked almost all of his human cloning research. "It's a setback from where we thought we were," says Rudolf Jaenisch, a stem cell expert at the MIT-affiliated Whitehead Institute for Biomedical Research in Cambridge, MA. "We thought [human cloning] was efficient, now we don't know."