Colossal Biosciences reported on March 4, 2025, that it had produced gene-edited mice with long, woolly and golden-brown coats as part of research connected to its plan to create elephants with woolly-mammoth traits. The work had not been peer reviewed, and researchers had not yet shown that the animals were more tolerant of cold.

The company used several editing approaches. Scientists altered fertilised mouse eggs or modified embryonic stem cells before injecting them into embryos and transferring those embryos to surrogate animals. They targeted nine genes associated with coat colour, texture, length, pattern or follicle development, using different combinations in different experiments. One method made as many as eight edits across seven genes at once.

Most targets were mouse genes already known to affect coats. Two were also present in mammoths and were altered in ways intended to make the mouse versions more mammoth-like. The team additionally disrupted a gene involved in fat metabolism that has been identified in mammoths and might contribute to adaptation to cold. The resulting animals were not mice carrying a collection of inserted mammoth genes.

Many experimental attempts did not produce pups. Those born showed varied combinations of coat characteristics, while average body mass was similar whether or not the fat-metabolism gene had been modified. Colossal chief science officer Beth Shapiro said editing efficiency differed among animals, although some carried all attempted changes. The company planned behavioural testing to examine cold tolerance.

Colossal studies ancient mammoth genomes alongside those of Asian elephants and says it wants to produce an elephant with selected mammoth characteristics. Chief executive Ben Lamm described the mouse results as validation of the editing approach but said they did not accelerate the company’s schedule. Colossal was aiming for a first calf by the end of 2028, a target not demonstrated by the mouse experiment.

Independent scientists recognised the technical work while challenging its broader meaning. Robin Lovell-Badge of the Francis Crick Institute said it did not explain the biological mechanisms behind the coat changes or establish cold-adapted physiology and behaviour. He also questioned whether de-extinction research was the best use of resources compared with preventing existing species from disappearing.

Tori Herridge of the University of Sheffield stressed that engineering an elephant would be much more complex. More genes would probably be involved, many remain poorly understood, and any surrogate would be an animal not normally used in experiments. The reported mice therefore demonstrated that multiple known coat-related edits could be combined. They did not establish mammoth de-extinction, confirm cold tolerance or resolve the scientific and ethical challenges of producing a mammoth-like elephant.