More than 20 years after Dolly comes Zhong Zhong and Hua Hua.

The twin long-tailed macaque monkeys are the first primates cloned using the same method that created the world’s most famous sheep in 1996—a method called somatic cell nuclear transfer, or SCNT. The twins’ genetic blueprints were swiped from fetal cells of another monkey. Researchers then popped the DNA into egg cells that they had also cleared of their DNA-containing nuclei. With a dash of compounds that spur embryo development, the reprogrammed cells developed into healthy baby monkeys in surrogate mother monkeys. The two were born about seven weeks ago in China and are developing normally so far, researchers reported Wednesday in the journal Cell.

Though the overall SCNT method is the same as what was used for Dolly, researchers struggled for years to tweak it to work in primates. The procedure is delicate and required a lot of optimization—not to mention DNA-swaps.

The researchers behind the cute clones, led by Zhen Liu of the Chinese Academy of Sciences, first tried using DNA from adult monkey cells. They created 192 embryos this way, implanting 181 of them into 42 surrogates, leading to 22 pregnant monkeys. But this resulted in the live birth of only two monkeys, both of which died within hours. Next, the researchers tried using DNA from fetal tissue. They created 109 embryos, implanted 79 of them into 21 surrogates, leading to pregnancy in six of them. Two female monkeys, Zhong Zhong and Hua Hua, resulted.















Qiang Sun, Mu-ming Poo, and the Chinese Academy of Sciences

The researchers attribute their success to new cell-imaging methods, tweaking the right mix of reprogramming compounds, and lots of practice.

The resulting clones are twins because they were created using DNA from the same fetal tissue. Their names stem from the Chinese word "Zhonghua," which means "Chinese nation" or "Chinese people."

The monkeys are a technical breakthrough for SCNT, but they're not the first cloned primates. That title goes to Tetra, a rhesus macaque born in 1999 and created with another cloning technique called "embryo splitting."

Still, Liu and colleagues are hopeful that the new cloning method may revive primate research and provide uniform genetic backgrounds that may make it easier to study and unravel genetic diseases. Research on monkeys and other primates has fallen out of favor among many scientists, particularly in the US. The research is costly and can create ethical dilemmas, and primates are also not perfect models of human disease. But the authors of the new study think their new clones could change things.

"Once we demonstrate the cloned monkey's usefulness in curing disease, I hope [Western societies] will gradually change their minds," co-author Mu-ming Poo told Science.

Cell, 2018. DOI: 10.1016/j.cell.2018.01.020 (About DOIs).

Images courtesy of Qiang Sun, Mu-ming Poo, and the Chinese Academy of Sciences