Scientists Find Fatal Limits to Animal Cloning After 20-Year Mouse Experiment

Japanese researchers discovered that repeatedly cloning mice over two decades leads to deadly genetic defects that accumulate with each generation. The study of 1,206 cloned mice showed that while the first 25 generations appeared healthy, fatal mutations eventually built up until the 58th generation died within days of birth.

Scientists in Japan have uncovered troubling evidence that animal cloning has serious biological limits after conducting a groundbreaking 20-year experiment with laboratory mice.

The research team created 1,206 cloned mice from a single female donor between 2005 and 2025, making new clones from each previous generation every three to four months. While the first 25 generations appeared normal and healthy, dangerous genetic defects began accumulating that eventually proved lethal.

By the 58th generation, the cloned mice—though appearing physically normal—died within days of being born due to the burden of accumulated genetic damage.

“No one has ever continued re-cloning for this long before. As a result, this is the first time we’ve discovered that repeated re-cloning eventually reaches its limits,” explained Teruhiko Wakayama, a developmental biologist at the University of Yamanashi who led the study published Tuesday in Nature Communications.

The findings challenge long-held assumptions about cloning technology and its potential applications. Scientists had previously believed that clones were perfect genetic copies of their donors.

“It was once believed that clones were identical to the original, but it has become clear through this study that mutations occur at a rate three times higher than in offspring born through natural mating,” Wakayama noted. “Because all these mutations continue to accumulate, mammals cannot sustain their species through cloning. This study has revealed one of the reasons why mammals, unlike plants and lower animals, cannot maintain their species through cloning.”

The researchers initially published promising results in 2013 covering the first 25 generations, concluding that indefinite cloning seemed possible. However, continued monitoring revealed a different story.

“At that time, we concluded that re-cloning could likely continue indefinitely. However, in that study, we did not examine the genetic sequences. We continued our research for 13 more years, and as a result, we discovered that our previous conclusion was incorrect—that is, there is a limit to re-cloning,” Wakayama said.

To understand what was happening at the molecular level, the team analyzed the complete genetic sequences of 10 clones from different generations. They discovered that serial cloning works like making photocopies of photocopies—each successive copy becomes more distorted than the last until the final result bears little resemblance to the original.

The deterioration became measurable in practical ways. Early-generation clones produced normal-sized litters of about 10 offspring when mated with regular male mice, matching typical reproduction rates. However, later generations had increasingly smaller litters as genetic damage mounted.

Starting with the 27th generation, researchers observed major chromosomal problems, including the loss of entire X chromosomes—critical genetic structures that females need two copies of for normal development.

The scientists used nuclear transfer technology, the same method that created Dolly the sheep in Scotland in 1996 and the first cloned mouse in Hawaii in 1998. This process involves transferring genetic material from a donor cell into an egg cell whose own nucleus has been removed.

“In cloning, all genes are passed on to the next generation, meaning that all defective genes are also passed on,” Wakayama explained, highlighting why sexual reproduction—which mixes genetic material from two parents—helps mammals avoid such problems.

The results have disappointed researchers who had hoped cloning technology could be refined for broader applications.

“We had believed that we could create an infinite number of clones. That is why these results are so disappointing. At this point, we have no ideas for overcoming this limitation. I believe we need to develop a new method that fundamentally improves nuclear transfer technology,” Wakayama concluded.