Even with all the modifications, the team was able to produce only two stem cell lines from 304 eggs subjected to the process, a success rate of 0.7%. One of those lines had an abnormal Y chromosome; the other was normal.
Other scientists were not particularly concerned with the inefficiency, however. When any technology is first developed, "it will be inefficient and not terribly practical," said molecular biologist Larry Goldstein of UC San Diego, who was not involved in the research. "The first computer was pretty slow. . . . You have to keep it in perspective."
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The team did not attempt to use the embryos to produce baby monkeys by implanting them in surrogate mothers, Mitalipov said.
James Byrne, lead author of the study, moved from Oregon Health & Science University to Stanford University over the summer and is now trying to replicate the work using human eggs and cells, said Renee Reijo Pera, director of human embryonic stem cell research at the Stanford Institute for Stem Cell Biology and Regenerative Medicine.
She noted that one of the major impediments to the research is obtaining an adequate supply of human eggs. "Availability will always be a major problem," she said.
The report is not the first to claim success at cloning a primate embryo. Korean researcher Hwang Woo-suk published two papers in the journal Science in 2004 and 2005 saying that he had cloned a human embryo.
The papers were later withdrawn after Hwang was accused of fabricating data, and other researchers were unable to replicate his results. Hwang lost his position at Seoul National University and faces criminal charges for fraud and misuse of research funds.
The editors of Nature took the highly unusual step of submitting biological samples from Mitalipov's lab to Australian researchers for confirmation.
Geneticist David Cram and his colleagues at Monash University analyzed DNA from the male macaque who served as a skin cell donor, the two females who donated the eggs and the stem cells themselves.
They concluded that "beyond any doubt" the stem cells came from cloned embryos.