Scientists in Japan have achieved a striking feat in reproductive biology. They created female clones from male mouse cells by excising the Y chromosome. The resulting mice developed normally, reached adulthood, and produced offspring when bred.
But this isn’t simple cloning. The work sidesteps long-standing barriers in mammalian sex determination. It opens fresh questions about conservation, genetic diversity, and the future limits of assisted reproduction.
Shogo Matoba at the RIKEN BioResource Research Center led the effort. His team developed a CRISPR tool they named Y-CUT. It targets sequences on the Y chromosome and triggers its elimination during early embryonic stages. The chromosome gets packaged into micronuclei and discarded. What remains is an XX cell line derived entirely from the original male genome.
The Technique and Its Immediate Results
The approach works in two contexts. First, applied directly to early-stage male embryos, Y-CUT efficiently removes the Y chromosome and allows female development. Second, the researchers combined it with optimized somatic cell nuclear transfer. They took nuclei from adult male cells, applied the editing, and transferred them into enucleated eggs. The process yielded both male clones and sex-reversed female clones from the very same male donor.
These females are genetically identical to the original male, apart from the missing Y chromosome, Matoba told MIT Technology Review. The animals appeared healthy. Many survived to adulthood. When crossed with the male clones produced in parallel, they gave birth to viable pups. Sexual reproduction started from a single male somatic genome.
The preprint, posted days ago on bioRxiv, describes the method in detail. “By establishing a method that enables robust Y-chromosome elimination in embryos, we provide a strategy to regulate sex in mice,” the authors write. “Offspring were successfully obtained from crosses between these male and sex-reversed female clones, demonstrating that this ‘dual-sex cloning’ strategy can initiate sexual reproduction solely from a male somatic genome.” (bioRxiv, August 2026).
Success didn’t come easily. Previous attempts at similar sex reversal in mammals stumbled over imprinting conflicts and developmental arrest. The Japanese group refined the nuclear transfer conditions and timed the CRISPR activity precisely. They avoided the widespread gene expression errors that once doomed cloned embryos.
Short. Simple. Effective. The females looked normal. They bred. Their litters showed no obvious defects in the first generation.
Yet the work builds on decades of incremental advances. Earlier Chinese experiments produced mice with two fathers by editing imprinting genes. Those bi-paternal pups reached adulthood but required complex manipulation of more than a dozen regions. (C&EN, January 2025). The new Japanese method focuses on chromosome-level removal rather than fine-tuned epigenetic rewriting.
And the implications stretch beyond the lab. Conservation biologists face shrinking populations of endangered species where only a few males remain. Dual-sex cloning could, in theory, generate breeding females from those males alone. It would expand the effective population size without introducing foreign genetics.
The preprint authors highlight exactly this point. They call dual-sex cloning “a valuable platform for securing mammalian reproduction and biodiversity from limited genetic resources.” Conservation groups have already begun informal discussions about adapting the technique for rhino or big-cat cell lines. No field trials exist yet. Regulatory and ethical reviews would take years.
Critics raise immediate concerns. Mice differ from larger mammals in placental biology and imprinting patterns. Scaling to primates or livestock might expose hidden incompatibilities. The females, though fertile, carry two X chromosomes from the original male. One X remains inactive, but subtle dosage effects could appear in later generations or under stress.
Matoba’s group reports high efficiency in Y elimination. Most treated embryos lost the Y chromosome completely. Survival rates after nuclear transfer improved with their optimized protocol, though exact figures remain tied to the full paper still under review.
So what does this mean for human medicine? The authors stay cautious. They make no claims about translating the work to people. Still, the demonstration that a male genome can sustain both male and female lineages challenges old assumptions about obligatory maternal contributions.
Previous cloning milestones delivered similar shocks. Dolly the sheep arrived in 1996 and forced society to confront reproductive possibilities. The 2010s brought mice with two fathers through elaborate stem-cell reprogramming. Each step revealed that mammalian development holds more flexibility than textbooks suggested.
This latest advance feels different. It doesn’t just combine genomes. It converts one genome into two sexes. The male donor’s cells give rise to brothers and sisters that can then mate. A closed reproductive loop emerges from a single individual.
But fertility comes with caveats. The paper notes that not every clone performed equally. Some females produced smaller litters. A fraction showed minor placental abnormalities common in cloned animals. Long-term health monitoring continues.
Experts outside the project express measured enthusiasm. “This adds a powerful new tool to the reproductive biology toolkit,” one stem-cell researcher said privately. Public commentary on X echoed the preprint’s excitement while warning against overhyping applications to endangered species or, worse, human reproduction.
The technique also revives old debates about cloning ethics. Animal welfare groups question the necessity of large-scale nuclear transfer experiments. Proponents counter that the work targets conservation crises where traditional breeding has already failed.
Practical barriers remain high. Nuclear transfer demands skilled technicians and expensive equipment. Y-CUT requires custom guide RNAs and careful delivery. Even with high success rates in mice, adapting the system to other species will demand fresh optimization.
Still, the proof of concept lands at an opportune moment. Climate change and habitat loss accelerate species declines. Gene banks hold frozen cells from males of many threatened animals. The new method suggests those samples could generate breeding pairs rather than just male clones.
Matoba and colleagues emphasize the basic science value first. Their system lets researchers study sex determination, X-chromosome inactivation, and imprinting with greater control. It isolates the effects of the Y chromosome by producing isogenic XX and XY animals from the same donor.
Future experiments could test whether the sex-reversed females transmit any subtle epigenetic memory of their male origin. Preliminary data suggest the X chromosomes behave normally, but deeper sequencing is underway.
The work arrives months after another landmark: fertile androgenetic mice created by editing seven imprinting control regions. That Chinese-led study showed two sperm genomes could produce viable adults when specific epigenetic marks were rewritten. (PNAS, June 2025). Together, these papers signal rapid progress in bypassing traditional parent-of-origin requirements.
Yet each success also exposes new complexities. Mammalian reproduction evolved tight controls for good reason. Disrupting them risks unexpected developmental trade-offs that may only surface after multiple generations.
For now, the Japanese team’s mice stand as the clearest demonstration yet that a single male genome can launch a self-sustaining population. The females look ordinary. Their behavior matches wild-type counterparts. Their pups carry the donor male’s genetics through both maternal and paternal lines.
Conservationists see potential. Ethicists see risks. Biologists see opportunity. The conversation has only begun.
One thing feels certain. The boundary between male and female contributions in reproduction has grown more porous. And the tools to explore that boundary keep improving.