Scientists have finally succeeded in using cloning to create human embryonic stem cells, a step toward developing replacement tissue to treat diseases but one that might also hasten the day when it will be possible to create cloned babies.

The researchers, at Oregon Health and Science University, took skin cells from a baby with a genetic disease and fused them with donated human eggs to create human embryos that were genetically identical to the 8-month-old. They then extracted stem cells from those embryos.

The embryo-creation technique is essentially the same as that used to create Dolly the sheep and the many cloned animals that have followed. In those cases, the embryos were implanted in the wombs of surrogate mothers.

The Oregon researchers, led by Prof. Shoukhrat Mitalipov, did not implant their human embryos and said they had no intention of doing so. They say their technique, in any case, would not lead to the birth of a viable baby. The same technique, tried in monkeys for years, never resulted in the birth of a cloned monkey, they said.