Cloning is most efficient using non-stem cells.

Fully matured cells can be used to clone animals; in fact, using such cells for this purpose may be more efficient than using stem cells, scientists report.

Since Dolly the cloned sheep was born in 1996, some scientists have speculated that the donor cells used to create her and other cloned animals were rare adult stem cells--immature cells that have the potential to create a multitude of other cell types.

To examine how a cell's maturity affects its usefulness for cloning, Xiangzhong (Jerry) Yang of the University of Connecticut in Storrs and his colleagues worked with three types of blood cells from a mouse: stem cells that produce all types of blood cells, more-mature cells that can make only a few blood cell types, and fully mature white blood cells called granulocytes that can no longer divide. All the cells were harvested on the same day.

Yang's team isolated the cells' nuclei and injected them into mouse eggs whose own nuclei had been removed. The researchers got a surprising result: About 35 percent of the fully mature cells produced embryos, whereas only 11 percent of the intermediate and 8 percent of the stem cells did.

Yang and his colleagues report in Nature Genetics, as published online Oct. 1, that the finding could eventually streamline therapeutic cloning. In that procedure, researchers make cloned embryos and then harvest stem cells from them for growing specific tissues.