A Japanese team took cells from a male mouse and ended up with female pups that grew up, mated, and had healthy young of their own. No gene that controls sex was edited. The Y chromosome was simply removed. The preprint, from the University of Yamanashi and Riken, went online on August 4, 2026. A little over four months earlier, another lab at the same university had published the conclusion that cloning eventually runs out of road.

Cutting the Y, or rather leaving it behind

The technique is called Y-CUT, short for Y chromosome elimination via centromere-targeted Cas9-induced cuts.

Cloning starts the usual way. The nucleus of a male cell goes into an egg that has had its own nucleus taken out. Then, before the embryo starts dividing in earnest, CRISPR-Cas9 makes several cuts in the repeating stretches of DNA that form the Y chromosome's centromere. The centromere is the handle the cell's machinery grabs to haul a chromosome into a daughter cell. Cut the handle enough times and nothing gets a grip. The Y reaches neither daughter cell. It is left in the cytoplasm as a micronucleus, and the cell disposes of it.

What is left is a cell with a single X and no Y, which in a mouse develops as a female. Her autosomes are identical to the donor male's. She is his clone in every respect except the chromosome that was thrown away.

The counts reported from the work are small but consistent. Cloning from a newborn male produced 13 pups, 10 of them female. From an adult male, cloning without Y-CUT produced seven pups, all male; with Y-CUT, 11 were born and 10 were female. Female clones also came out of cryopreserved cells, and when those females were mated with male clones from the same donor, the pairs produced healthy offspring. Genome checks on six of the XO females turned up nothing abnormal, though six is a small number and the paper has not been through peer review.

Back in 2009, one female turned up by accident among 27 clones made from male mouse cells. That was luck. This is a procedure.

Monika Ward, who studies reproductive biology at the University of Hawaii, told MIT Technology Review that "no one has done this before."

The dead end this technique is answering

A little over four months before the preprint appeared, on March 25, 2026, the University of Yamanashi and the Radiation Effects Research Foundation announced the result of an experiment that had been running for about two decades: clone a mouse, then clone the clone, and keep going. They got to 1,206 cloned mice across 58 generations. Then every animal in the 58th generation died.

Harmful mutations had started piling up around the 27th generation, and the cloned line was accumulating mutations at roughly three times the rate of naturally bred mice. We covered that study when it came out, here. Its message was blunt: you cannot maintain a mammal indefinitely by copying it. Something has to reset the genome, and the thing that resets it is sex.

Read against that, Y-CUT stops looking like a stunt and starts looking like a plan. If copying degrades, then the useful move is not to copy better. It is to copy once, get both sexes out of the single genome you have, and hand the job back to ordinary reproduction. A male and a female clone from the same donor can breed, and their offspring are made the way mammals have always made offspring.

Takashi Ishiuchi, an associate professor at the University of Yamanashi who co-led the work, put the ambition plainly to MIT Technology Review: "There's a fixed concept in our scientific field that we need both females and males for reproduction. I think we could change this concept."

Nuclear transfer took root at Yamanashi by way of Hawaii. The first mouse cloned from an adult body cell, Cumulina, was born on October 3, 1997, in Ryuzo Yanagimachi's lab at the University of Hawaii at Manoa, with Teruhiko Wakayama at the bench. The method became known as the Honolulu technique. Wakayama later moved to Yamanashi, where the 58-generation experiment ran. Mouse nuclear transfer has been a Japanese specialty for close to thirty years, and Y-CUT sits at the end of that lineage.

The other Japanese route: eggs from male cells

In March 2023, a team led by Katsuhiko Hayashi, now a professor at Osaka University, reported mouse pups with two fathers. When male skin cells were reprogrammed into iPS cells, about 6% of them dropped the Y chromosome on their own and became XO. The researchers then made those cells duplicate their remaining X, producing XX cells, and grew them into eggs. Of 630 embryos transferred, seven pups were born, a success rate of just over 1%.

So one Japanese lab makes eggs out of male cells, and another makes a whole female animal out of a male cell. Shogo Matoba of the Riken BioResource Research Center, who co-led the Y-CUT work, called the relationship between the approaches "a complementary story." Hayashi's route delivers gametes without needing a female to exist. Y-CUT delivers a female who then produces her own eggs the ordinary way, for as long as she lives.

You still need a female to start

Y-CUT does not escape the constraint at the root of cloning. Every attempt begins with an egg that has had its nucleus removed, and that egg has to come from a female of the same species or a close relative. If a species is down to males, someone still has to supply the eggs. This is where Hayashi's route from the previous section would bite. But it runs at just over 1% in mice, so for now it is a candidate for the next move rather than an answer.

The second limit is biological. XO females breed fine in rodents. In most other mammals, an XO female tends to be infertile, which means the technique as demonstrated does not simply port to a rhino or a bird. The honest near-term scope is rodents. Roughly 355 rodent species are classified as endangered or vulnerable.

The mirror image of the problem sits in Kenya. Two northern white rhinos are alive, the females Najin and Fatu, and neither can carry a pregnancy. As of April 2026, 39 embryos had been produced and six embryo transfers had been attempted, none of which led to a lasting pregnancy. That is a species with no males and no shortage of eggs, which is the case Y-CUT cannot help with. What it hints at instead is the value of the frozen cells sitting in cell banks around the world, most of which came from animals that happened to be male.

Ben Novak, lead scientist at the conservation organization Revive & Restore, puts the hope in the growing set of tools rather than in any one result: "It's really exciting to see more diverse tools being developed. There are so many different scenarios in which they could be used for rare and endangered species."

How Japan actually read the news

The Japanese coverage picked up 113 bookmarks on Hatena Bookmark, and the comments split along three lines that had almost nothing to do with each other.

One group went straight to jokes about turning male humans female, which says more about Japanese internet humor than about the paper. A second group got technical fast, asking why the researchers removed the whole Y rather than knocking out the single sex-determining gene on it, and answering each other with the point that the Y also carries genes needed for sperm production, so taking the whole thing out is cleaner if you want fertility. A third group was uneasy: if the animal you release is one whose chromosomes you edited, is that still species conservation, or is it something else wearing conservation's clothes?

That last question is the one the field will have to answer in public, and it will not be settled by mouse counts. There is also a reader who simply asked which is better for reviving endangered species, this or making eggs from iPS cells. The past half-year of Japanese cloning research suggests the answer is that the question is wrong, and both will be needed.

Japan has spent nearly thirty years being very good at moving nuclei between mouse eggs, and in the space of one spring and summer it published both the limit of that skill and a way around it. Is cloning something your country debates as conservation, as agriculture, or mainly as science fiction?

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