NEW YORK -- For the first time, researchers have used the cloning method that produced Dolly the sheep to create two healthy monkeys, bringing science an important step closer to being able to do the same with humans. Since Dolly's birth in 1996, scientists have cloned nearly two dozen kinds of mammals, including dogs, cats, pigs, cows and polo ponies, and have also created human embryos with this method.

But until now, they have been unable to make babies this way in primates, the category that includes monkeys, apes and people. 

"The barrier of cloning primate species is now overcome," declared Muming Poo of the Chinese Academy of Sciences in Shanghai.

He and colleagues announced their success with macaques in a paper released Wednesday by the journal Cell. The female baby monkeys, about 7 and 8 weeks old, are named Zhong Zhong and Hua Hua.

"It's been a long road," said one scientist who tried and failed to make monkeys and was not involved in the new research, Shoukhrat Mitalipov of Oregon Health & Science University. "Finally, they did it."

In principle, Poo said, the feat means humans can be cloned. But he said his team has no intention of doing that. Mainstream scientists generally oppose making human babies by cloning, and Poo said society would ban it for ethical reasons.

Instead, he said, the goal is to create lots of genetically identical monkeys for use in medical research, where they would be particularly valuable because they are more like humans than other lab animals such as mice or rats.

The process is still very inefficient — it took 127 eggs to get the two babies — and so far it has succeeded only by starting with a monkey fetus. The scientists failed to produce healthy babies from an adult monkey, though they are still trying. Dolly caused a sensation because she was the first mammal cloned from an adult. 

The procedure was technically challenging. Essentially, the Chinese scientists removed the DNA-containing nucleus from monkey eggs and replaced it with DNA from the monkey fetus. These reconstituted eggs grew and divided, finally becoming an early embryo, which was then placed into female monkeys to grow to birth.

The scientists implanted 79 embryos to produce the two babies. Still, the approach succeeded where others had failed. Poo said that was because of improvements in lab techniques and because researchers added two substances that helped reprogram the DNA from the fetus. That let the DNA abandon its job in the fetus, which involves things like helping to make collagen, and take on the new task of creating an entire monkey.

The Chinese researchers said cloning of fetal cells could be combined with gene-editing techniques to produce large numbers of monkeys with certain genetic defects that cause disease in people. The animals could then be used to study such diseases and test treatments. The researchers said their initial targets will be Alzheimer's and Parkinson's.

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