For example, in 2016, scientists at the Institute of Neuroscience in Shanghai created monkeys with a mutated form of the gene MECP2, which is linked to autism spectrum disorder. The genetically modified creatures ran in circles and avoided other monkeys—behaviors that may echo children with the MECP2 mutation.

Yet, says Poo, the monkeys led to “very little” progress in understanding the disorder. “It is a feasibility demonstration that we can manipulate genes in monkeys, but it’s not a disease model,” he says. Different monkeys had copies of the gene in different parts of their genome, and their resulting behaviors varied. The technique used to create the MECP2 monkeys did not give scientists enough control. Similar problems can arise with CRISPR, a gene-editing technique that has been widely adopted in different organisms across the world. You can’t test new drugs on monkeys whose manifestations of disease are so different from one another.

With cloning, it is possible to genetically modify monkey cells in a dish, grow the cells in large numbers, and then create clones from them. The last step requires taking DNA out of the genetically modified cells and putting it into an egg—a procedure called somatic-cell nuclear transfer. Zhongzhong and Huahua were created this way, as was Dolly the sheep. (In 1999, scientists created Tetra the cloned monkey by splitting an eight-cell embryo into two, similar to the way twins are normally formed. This technique can create only a limited number of genetically identical monkeys.)

Unlike previous groups who have tried to clone primates unsuccessfully, Qiang Sun at the Institute of Neuroscience in Shanghai added two complex molecules after transferring the DNA. These molecules seem to alter the expression of genes—in particular, by unblocking genes necessary to reprogram a differentiated cell (whose fate has already been determined) into a stem cell (that can turn into anything). The molecules had been used before in other animals like mice but never successfully in primates.

The DNA from which Zhongzhong and Huahua were cloned came from fetal fibroblasts, a type of cell in connective tissue. The team also tried using DNA from adult-monkey cells, but the only two monkeys cloned this way died shortly after birth.

The cloning efficiency is still low—though comparable to that in other animals. To create Zhongzhong and Huahua, the team transferred fibroblast DNA into 127 eggs. Of those, 109 developed into embryos, and 79 were transferred into 21 surrogate monkey mothers. There were six pregnancies and just two live births. Given these numbers, cloning genetically modified monkeys will remain very expensive.

Cloned and genetically modified monkeys, says Christopher Navara, a biologist at the University of Texas at San Antonio, “could be very useful for studies of some diseases, but less so for others.” Childhood disease could be studied in young monkeys immediately. Aging-related disease like Alzheimer’s or Parkinson’s would take decades, though. Crab-eating macaques, the species to which Zhongzhong and Huahua belong, live to 25 to 30 years.