# Woolly mouse

The woolly mouse refers to a variety of genetically modified laboratory mice developed in 2025 by Colossal Biosciences as part of efforts toward the de-extinction of the woolly mammoth. The mice feature mammoth-inspired traits which include woolly coats and other genetic modifications aimed at cold tolerance.

## Background

Colossal Biosciences, an American biotechnology company, initiated the woolly mouse project as part of their broader woolly mammoth de-extinction program. The company made efforts to restore woolly mammoths through genetic engineering of Asian elephants by introducing mammoth-like traits that would allow the resulting species to survive in cold environments.
The company proposed that herds of genetically modified elephants with mammoth-like traits could help mitigate climate change through their impact on Arctic ecosystems. According to the company's hypothesis, these creatures would graze in ways that promote grassland development in tundra regions and reduce permafrost thaw, thus decrease the release of carbon dioxide from permafrost melt.
The research was announced in March 2025, with the company stating their intention to produce the first genetically modified elephant calf with mammoth characteristics by the end of 2028.

## Development

The research team employed genome editing techniques to develop the woolly mouse, including the direct genetic modification of fertilized mouse egg and the modification of embryonic mouse stem cells, which were then injected into mouse embryos. This was followed with the implantation of modified embryos into surrogate mice.
The scientists targeted nine specific genes associated with hair characteristics (color, texture, length, pattern, and follicle development). Seven of the genes were selected based on previous knowledge of their influence on mouse coats, with the modifications designed to produce traits that would resemble mammoth features, such as golden-colored fur. This included FGF5 (fibroblast growth factor 5), that is involved with the hair growth cycle, and MC1R (melanocortin 1 receptor), involved in the regulation of melanin production. Two of the targeted genes had direct mammoth counterparts believed to contribute to the species's woolly coat. Additionally, the team modified a gene linked to lipid metabolism in mice that also existed in mammoths, hypothesizing that the modification might enhance cold adaptation.
While many of the experimental embryos did not develop into viable pups, all successfully birthed individuals were still alive as of March 2025, according to Colossal CSO Beth Shapiro. The successfully born mice exhibited woolly coats with elongated hair of about 5 centimetres (2.0 in) in length, bearing a golden-brown coloration. The efficiency of genetic editing varied among specimens, with many individuals displaying 100% of the attempted genetic modifications. Notably, mice with the modified fat metabolism gene showed no significant difference in average body mass compared to those without this modification. As of the announcement, the team had not yet conducted behavioral tests to evaluate cold tolerance in the modified mice, though they expressed plans to do so in the coming months.

## Reception

Scientific reception to the project was generally mixed. Robin Lovell-Badge, head of the Francis Crick Institute's developmental genetics and stem cell biology fields, acknowledged the technical achievements involved in the genetic modification procedures, but highlighted several limitations. He stated that the research did not elucidate the mechanisms through which the genetic modifications produced different hair types or whether the modified mice actually exhibited enhanced cold tolerance, and did not address the substantially greater complexity involved in de-extincting mammoths. Lovell-Badge questioned whether the financial resources directed toward de-extinction efforts might be better utilized in preventing extinctions of currently endangered species.
University of Sheffield paleontologist Dr. Tori Herridge emphasized that engineering a woolly mammoth-like elephant presents significantly greater challenges compared to mice. These included elephants bearing a much larger number of genes involved in the phenotype, having less understood genetic mechanisms, as well as the potential complications related to using an uncommon experimental animal as a surrogate.