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

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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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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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