WASHINGTON (AP) — U.S. scientists used a cloning technique to create the type of customized stem cells that show promise for treating disease. But the first-of-its-kind result comes with a big hitch.

The newly created stem cells have too much genetic material to function properly. They include DNA from two people, not just the patient the cells were meant to match. Scientists still have to figure a way around that before they can use the cells to develop genetically-matched tissue for transplant to treat various diseases.

The research involved injecting DNA from skin cells of a volunteer into donor eggs. Normally in cloning, the egg's genetic material is removed first, but when researchers tried that it didn't work, said study lead author Dieter Egli, a senior research fellow at the New York Stem Cell Foundation. This new method only worked when the donor's DNA was left inside the egg and the volunteer's genetic material was added. That meant the result had 69 chromosomes. That is 23 too many.

Because of that, Egli said, "I could never imagine that those cells could make a viable human being. That's simply out of the question."

The researchers were not trying to clone to create a human, however; they just wanted to produce stem cells.

Egli said he and his colleagues are trying several different approaches to overcome the "technical barrier" of too much DNA with this technique. What's important, he said, is that it shows researchers can use this method to turn a person's own cells into potent stem cells, something that has been demonstrated before in animals. The study was published online Wednesday in the journal Nature.

Researchers started with 270 eggs and eventually created two stem cell lines, according to study co-author Scott Noggle, director of the New York Stem Cell Foundation lab where the work was done.

There is also another promising method to create personalized stem cells that does not involve embryos. That technique reprograms skin cells to turn into stem cells. Egli says that method has problems, too.

The New York group's work "is a major step forward scientifically," but the problem of the egg's leftover DNA is a serious limitation, said George Daley of the Harvard Stem Cell Institute, who was not involved in the research. Daley said he thinks the alternative method of reprogramming cells will end up being more useful in the long run. But Egli's finding will help the competing method improve, Daley said.

Arthur Caplan, director of the Center for Bioethics at the University of Pennsylvania, said this new work shows how hard it would be to use cloning to create people. That avoids a major ethical concern with embryonic stem cells, he said.