It is a technique called nuclear transfer -- the same used to create Dolly the sheep -- and until Mitalipov's research there had been skepticism over whether a primate could be cloned in the same manner.

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MRI, first developed 30 years ago, is based on a well established scientific technique, nuclear magnetic resonance, which uses the interaction of magnetic fields with the spin of the nuclei of atoms to provide detailed information on the constituents of chemicals and biological materials.

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They were cloned using a double nuclear transfer method similar to the technique used to get Dolly.

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In mice, Daley and colleagues have shown that stem cells derived from the nuclear transfer of cells to make embryos -- the technique described in Mitalipov's paper -- were indeed closer to natural embryo stem cells than induced pluripotent stem cells.

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Instead of generating embryonic stem cell lines via nuclear transfer, Hwang's group produced the stem cells from days-old embryos, a technique that had already been established by James Thomson at University of Wisconsin in 1998.

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