From what I'v read, Embryonic stem cells are the stem cells creating tumors.

http://bluecrabboulevard.com/2006/10...m-cells-tumor/
October 22, 2006

A very disquieting news report that should really bother those who are strong proponents of embryonic stem cell research. It is very preliminary, but it is also very frightening.

Steven Goldman and colleagues at the University of Rochester Medical Center in New York said human stem cells injected into rat brains turned into cells that looked like early tumors.

Writing in the journal Nature Medicine, the researchers said the transplants clearly helped the rats, but some of the cells started growing in a way that could eventually lead to a tumor.

Various types of cell transplants are being tried to treat Parkinson's disease, caused when dopamine-releasing cells die in the brain.

This key neurotransmitter, or message-carrying chemical, is involved in movement and Parkinson's patients suffer muscle dysfunction that can often lead to paralysis. Drugs can slow the process for a while but there is no cure.

The idea behind brain cell transplants is to replace the dead cells. Stem cells are considered particularly promising as they can be directed to form the precise desired tissue and do not trigger an immune response.

Goldman's team used human embryonic stem cells. Taken from days-old embryos, these cells can form any kind of cell in the body. This batch had been cultured in substances aimed at making them become brain cells.

Previous groups have tried to coax stem cells into becoming dopamine-releasing cells.

Goldman's team apparently succeeded and transplanted them into the rats with an equivalent of Parkinson's damage. The animals did get better.

But the grafted cells started to show areas that no longer consisted of dopamine-releasing neurons, but of dividing cells that had the potential to give rise to tumors.

The researchers killed the rats and are very worried about the outcome they saw. This one ought to give a lot of people pause. There is a lot that is not at all understood here. It is really unfortunate that there is a political debate about science that appears to be not quite ready for prime time.
UPDATE: Additional coverage in the Washington Post this morning:

"The behavioral data validate the utility of the approach. But it also raises a cautionary flag and says we are not ready for prime time yet," said lead researcher Steven A. Goldman, a professor of neurology and neurosurgery at the University of Rochester Medical Center.

Goldman said he suspected that with modest changes in technique, researchers will be able to keep the benefits of the treatment while eliminating or reducing the chances of getting the cancerlike growths. But he conceded that much more basic research would have to be done before scientists ? or regulators ? were likely to be convinced of the approach's safety.