WASHINGTON (Reuters) - U.S. researchers said on Sunday they had found a more efficient way to clone mice, and said their experiment solved a basic question about cloning science -- whether it truly is possible to clone animals from mature cells.

Dolly the sheep made headlines around the world in 1997 because she was the first mammal to be cloned from an adult cell -- a cell taken from a grown animal. Animals had been cloned using cells from fetuses and embryos, but not grown animals.

But some scientists argued that truly mature tissue is too old to be re-generated, and that Dolly and the hundreds of cloned cattle, pigs and other creatures that followed here were in fact created using stem cells or stem-like cells by accident.

These master cells of the body retain an ability to form various tissues, and they are not always easy to pick out from the cells around them.

Dr. Jerry Yang of the University of Connecticut noted that cloning is still very difficult to do. Only about 2 or 3 out of 100 tries generally works.

"This was seen as circumstantial evidence that it was stem-like cells that succeeded in cloning," Yang said in a telephone interview.

"The question is important because the success rate for reproductive cloning is still quite low."

Yang, Dr. Tao Cheng, of the University of Pittsburgh, and colleagues cloned mice using fully differentiated, or mature, white blood cells called granulocytes.

Writing in the November issue of Nature Genetics, they said they used somatic cell nuclear transfer, in which the nucleus from a cell of the animal to be cloned is injected into an egg whose nucleus has been removed.

This process works very poorly in mice and usually a two-step procedure is needed -- first growing tiny embryos, then removing their embryonic stem cells to generate mouse pups.

Yang's team tried cloning using the blood cells at various levels of development -- from the stem cells stage through full maturity, called full differentiation.

"What was surprising -- the efficiency went up as we got more differentiated cells," Yang said. "That was very, very surprising, very shocking to us."

Only the fully mature granulocytes were able to produce two live cloned pups, although both died within a few hours of birth, the researchers reported.

"Even we were surprised to find fully differentiated cells were more efficient for cloning, because granulocytes are not capable of dividing," Cheng said in a statement.

"In fact, we repeated our experiments six times just to be sure. Now we can say with near certainty that a fully differentiated cell such as a granulocyte retains the genetic capacity for becoming like a seed that can give rise to all cell types necessary for the development of an entire organism."

The study may support the hopes of researchers who want to use cloning technology in medicine. Supporters of so-called therapeutic cloning want to some day be able to take a single cell from a patient, perhaps a skin cell, and use it to generate tailor-made tissue or organ transplants.

To do so, fully mature cells must maintain the ability to regress in this way and be reprogrammed.