Researchers create mice born from 2 male parents

A research group led by Wei Li at the Chinese Academy of Sciences has created mice born from two male parents and capable of reaching adulthood. This builds on previous work from the group in which the researchers had created viable mice born from two female parents (Cell Stem Cell 2025, DOI: 10.1016/j.stem.2025.01.005).

While a few other attempts at creating bipaternal mice have been reported, in those cases the mice failed to reach adulthood. In this new study, a handful of the bipaternal mice were capable of reaching adulthood, but they were sterile and had growth defects.

That the mice reached adulthood at all was possible thanks to genetic editing of genes involved in what is known as the imprinting process. Typically, in mammalian reproduction, offspring are born with two copies of most genes—one passed down from the mother and another passed down from the father. Imprinting is a process in which the expression of each gene copy is determined by the sex of the parent. Not all genes go through imprinting, but for those that do, imprinting is required for proper development of the offspring.

In a written statement provided to C&EN, the authors of the paper say that imprinting is a “fundamental barrier to unisexual reproduction in mammals” and that “even when constructing bi-maternal or bi-paternal embryos artificially, they fail to develop properly and stall at some point during development,” due to the lack of proper imprinting.

The research group used the Nobel Prize–winning gene-editing system CRISPR to edit the DNA of 20 genes in male haploid embryonic stem cells, which are stem cells that contain only one copy of each chromosome. Most of these edits were deletions of DNA sequences called imprinting control regions that help regulate the imprinting process and are located near many imprinted genes.

After editing, the research group injected a modified cell along with a sperm cell into an egg cell whose nuclei had been removed. The researchers then transferred the resulting bipaternal embryos to a surrogate mother.

Martin Leeb, a stem cell biologist at the Max Perutz Labs Vienna who was not affiliated with the study, says that the work is “impressive in its technical complexity” and that “it’s really not a small feat to make those 20-knockout stem cells while maintaining the developmental potential in a way that they can serve as surrogates for fertilization purposes.”

In 2011, Leeb published a study in Nature detailing the generation of the first mouse haploid embryonic stem cells—a technological advance used in this new work (DOI: 10.1038/nature10448). Leeb is excited about the paper and not entirely surprised that the creation of bipaternal mice came only a decade after his work, but he is surprised that doing so didn’t require even more effort. “I would have personally thought it probably requires even more genetic engineering to get these bipaternal mice born,” he says.

The authors of the paper say that this breakthrough in the manipulation of imprinting genes may have a role in therapeutic cloning, in which researchers create an artificial embryo with the same DNA as the donor for the purpose of harvesting embryonic stem cells. Therapeutic cloning has not been performed using human cells, because of ethical concerns and technical limitations. The researchers say that the complexities of the imprinting process is one of those limitations.

The researchers do claim, however, that they “aim to extend the experimental approaches developed in this study to larger animals, including primates,” specifically citing monkeys as another animal model, though they note this will take time, as the imprinted genes in monkeys are different from those in mice.

Leeb says that he has no major ethical concerns about this work being performed in mice but that continuing this work in primates raises major technical and ethical concerns. “I can’t imagine that these experiments could ever be replicated in humans,” he says. “It would be unethical and against the law. It is not allowed, and it should not be allowed.”