A form of cloning has been used to create personalised embryonic stem cells in humans, say researchers.
Genetic material was taken from an adult skin cell and transferred into a human egg. This was grown to produce an early embryo.
Stem cells have huge potential in medicine as they can transform into any other cell type in the body.
However, the stem cells formed contained chromosomes from both the adult and the egg cells.
The technique used - somatic cell nuclear transfer - shot to fame in 1997 when Dolly the sheep, the first mammal to be cloned from an adult cell, was unveiled to the world.
A South Korean scientist, Hwang Woo-suk, had claimed to have created stem cells from cloned human embryos, but was found to have faked the evidence.
The lead researcher at the New York Stem Cell Foundation Laboratory, Dr Dieter Egli, said there was "a great question mark" about whether the cloning technique could be reliably used in humans.
He said other "groups had tried before, but failed".
Writing in the journal Nature, he said his group had also failed using traditional techniques.
When they removed the genetic material from the egg and replaced it with the chromosomes from a skin cell, the egg divided but failed to go past the 6-12 cell stage.
However, when they left the egg's own genetic material in place and added the skin chromosomes, the egg developed. It reached the blastocyst stage, which can contain up to 100 cells and is the usual source of embryonic stem cells.
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