Human cloning easier than thought?

DURHAM, North Carolina (CNN) -- Cloning "may be less complicated in humans than in sheep" because of a subtle genetic difference between humans and most other animals, according to researchers at Duke University.

The finding, published Wednesday in the journal Human Molecular Genetics, suggests that humans and primates might not be susceptible to the danger of clones growing dangerously large in the womb.

"This is the first concrete genetic data showing that the cloning process could be less complicated in humans than in sheep," said Keith Killian, a molecular evolutionist at Duke University Medical Center who is the first author of the study.

Some researchers who say it is not safe to clone humans have told CNN they fear those who do will use the Duke study to validate their claims that cloning a human will not create a grotesquely disfigured baby.

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Dr. Randy Jirtle, a professor of radiation oncology at Duke who also authored the study, said: "We don't clone here, we're not planning to clone here and we're not advocating human cloning. We're just interested in the phenomenon of genomic imprinting."

Kevin Eggan, a researcher at MIT's Whitehead institute studies cloned mice. He called the Duke study "interesting from the perspective of the evolution of imprinting genes." But he pointed out there is no proof that abnormally large babies are born as a result of this one genetic difference.

The researchers said most mammals, especially the most commonly cloned animals such as cattle, mice and sheep, inherit one active insulin-like growth factor II receptor (IGFR2R) gene. Humans and primates, however, carry two such active genes, they said.

IGFR2R is believed to contribute to large-offspring syndrome, which causes cloned animals to grow grotesquely large in the womb and poses a danger to the mother and the fetus.

That syndrome has been the basis for debate about the feasibility of human cloning, since most cloned animals do not survive or suffer severe respiratory, cardiovascular and immunological deficiencies.

Dr. Don Wolf, who cloned two rhesus monkeys at the Oregon Regional Primate Research Center two years ago, said the Duke research shows a "very interesting approach to explaining why there are differences in species."

But Wolf cautioned that the study should not interpreted to mean it is safe to clone a human.

Killian and Jirtle studied genetic markers of animals on the evolutionary ladder from lemurs -- the bottom rung of primates -- up to humans and did not see any imprinted IGFR2R genes.

They said they believe approximately 70 million years ago humans and primates lost the ability to turn off the gene and so are not at risk of large offspring syndrome in their clones.

Eggan said his lab has a "four times normal size" mouse clone, which has normal IGFR2R genes. He said there are other factors that can contribute to abnormal development in clones.

-- CNN Medical Producer Miriam Falco and CNN.com editor Jonathan D. Austin contributed to this report.