We are now one step closer to same-sex reproduction: Scientists in China have produced healthy offspring from two mother mice.

According to a study published Thursday in the journal Cell Stem Cell, a team of researchers from the Chinese Academy of Sciences (CAS) used stem cells and gene editing to produce over two dozen healthy mice from female parents. Some of the engineered offspring even went on to have babies of their own.

While it is possible for some reptiles, amphibians, and fish to reproduce without two parents of the opposite sex, achieving this in mammals has proved challenging even with fertilization technology, the journal wrote in a press release sent to Sixth Tone. But for the CAS researchers, 210 embryos made from the DNA of two mothers yielded 29 live mice.

“We were interested in the question of why mammals can only undergo sexual reproduction,” co-senior author Zhou Qi was quoted as saying in the press release. “So we tried to find out whether more normal mice with two female parents, or even mice with two male parents, could be produced using haploid embryonic stem cells.”

Stem cells are widely used for genetic studies because they have the potential to develop into any other type of cell. “Haploid” cells — such as eggs and sperm — have half the number of chromosomes found in nonreproductive cells.

In mammals, offspring that lack genetic material from either a mother or a father experience abnormalities during development, and usually end up being nonviable. In 2004, scientists in Japan produced the first mice from two mothers after deleting imprinted genes. But in this study and many since, the offspring exhibited defects, said Zhou, and the experimental methods were impractical and difficult to perfect. According to Li Wei, another co-senior author, the defective mice were smaller than normal, or demonstrated poor motor skills.

Key to the success of the latest study, the researchers believe, is the use of haploid embryonic stem cells (ESCs), whose DNA contains less imprinting programming that must be deleted. In normal development, imprinting means crossover between the parents’ DNA, so that the offspring shares characteristics of both the mother and father. But genetic crossover between cells from same-sex parents presents opportunities for things to go wrong. After deleting three imprinting regions from the haploid ESCs of one mother, the CAS researchers injected them into eggs from the other mother.

The researchers also tried to produce viable offspring from two father mice via a more complicated process requiring the deletion of seven gene-imprinting regions rather than three, but they were much less successful: 1,023 two-father embryos yielded just 12 living mice. Though none survived for longer than 48 hours, the researchers say they’re hopeful of improving the process, and of one day raising two-father offspring to adulthood.

Despite having success with mice, Li said the same technique cannot easily be applied to other mammals — such as humans — since each species has a unique set of problematic imprinted genes, and identifying these takes time.

“We realize that the results of this study might cause people to wonder about same-sex reproduction in humans,” Li told Sixth Tone. “While we don’t deny that this might be possible in the future, we think it’s dangerous to apply our research to humans, and we absolutely oppose such attempts.” The goal of CAS experiment, Li added, was to study gene imprinting and how it could potentially be used to make humans healthier.

Yet in spite of the study’s practical limitations and singular purpose, it may well be received as a beacon of hope for the future of genetic and reproductive studies. “This research shows us what’s possible,” Li said.

Editor: David Paulk.

(Header image: A mouse born from the DNA of two mothers is seen in a lab at the Chinese Academy of Sciences in Beijing, Sept. 28, 2018. Courtesy of Cell Stem Cell)