Chinese researchers have produced the first two monkey clones ever made using the method that created Dolly the sheep, in a landmark accomplishment that could open the door to new methods of customizable gene experimentation.

The identical long-tailed macaques were born six and eight weeks ago, and show every sign of being normal for monkeys their age, according to findings presented in the journal Cell on Wednesday by researchers from the Chinese Academy of Science Institute of Neuroscience.

The two monkeys have been named Zhong Zhong and Hua Hua, in a tongue-in-cheek nod to their country of origin. Their names are actually split-up, doubled forms of the word “Zhonghua,” which means “Chinese.”

The monkeys are not the first ones ever to be cloned, but they are the first cloned primates created through somatic cell nuclear transfer, the same process used to clone Dolly the sheep in 1996. The first-ever monkey clone was produced in 1999 through embryo splitting – a method that duplicates the process through which identical twins are formed.

Somatic cell nuclear transfer consists of removing the nucleus from an egg and replacing it with a differentiated cell, such as a skin cell, from the donor being cloned. The egg then uses the genetic blueprint from the implanted cell to grow an identical life form inside a surrogate monkey mother.

Although Dolly was cloned using a sheep’s mammary gland cell, the monkey cloners had more success using a fibroblast tissue cell from a macaque fetus. Other cloning attempts were made using adult macaque cells, but those clones only lived for a few hours after birth.

The method has worked on cows and mice in the past, but monkeys proved to be more difficult.

The Chinese study authors say they ultimately cracked the code to cloning monkeys by re-activating suppressed genes in the differentiated donor cell, which allowed the embryos to develop normally inside a surrogate mother.

Genetically uniform monkey-testing

Senior study author Quang Sun touted the successful cloning project as a boon for future comparative studies, because it will potentially allow researchers to create multiple test subjects that are genetically identical, except for one or two genes.

“You can produce cloned monkeys with the same genetic background except the gene you manipulated,” Sun, director of the Chinese Academy of Science Institute of Neuroscience, said in a news release. He suggests the process can be used to examine the influence certain genes have on various brain diseases, metabolic disorders and cancers. One monkey could be used as a baseline example, while another with a tweaked gene could be compared against the first.

“There are a lot of questions about primate biology that can be studied by having this additional model,” Sun said.

Sun said there was “much failure” involved in his team’s cloning attempts, and that it took three years for lead study author Zhen Liu to perfect his technique with the nuclei.

“It takes a lot of practice,” study co-author Mu-Ming Poo said. “The faster you do it, the less damage to the egg you have, and Dr. Liu has a green thumb for doing this.”

The research team is expecting more monkey clones to be born in the coming months.

The study authors said their work adheres to strict international guidelines around animal research, as laid out by the United States National Institutes of Health.

“We are very aware that future research using non-human primates anywhere in the world depends on scientists following very strict ethical standards,” Poo said.