section_path: Development
section_name: Development
paragraph_text: 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.