Scientists have deliberately turned male mouse embryos into females for the first time. A team based in Japan used a CRISPR-based approach to remove the Y chromosome from male cells and create female clones of male mice. “No one has done this before,” says Monika Ward, a reproductive biologist at the University of Hawaii. The findings were published in a preprint paper shared on bio Rxiv earlier this month.

Takashi Ishiuchi and Shogo Matoba of the University of Yamanashi and Riken Bio Resource Research Center developed the technique, called Y-CUT, which targets a section of the Y chromosome. Treated XY embryos transferred to surrogate mice gave birth to female pups with XO chromosomes that grew up healthy and fertile. The researchers also created female clones from cryopreserved male cells, suggesting potential for conservation.

The approach could help rescue endangered species where only males remain, though it requires hollowed-out egg cells. Complementary techniques, such as turning male cells into egg cells (demonstrated in 2023) or inserting rat chromosomes into mice, may expand applications. “There are so many different scenarios in which they could be used for rare and endangered species,” says Ben Novak of Revive & Restore.