Sometimes, man-made mutants aren’t the writhing, creeping horrors from movies but instead a couple of extra-fluffy mice.

Colossal Biosciences has taken a significant step toward its goal of “resurrecting” the woolly mammoth by successfully creating genetically modified mice with traits resembling those of the extinct Ice Age giants. But what’s the science behind this process?

The goal for this experiment seems to be to identify genes that code for the mammoth’s key traits — including a golden, shaggy coat and increased lipid metabolism — then switch on and off certain genes in mice so these traits can be added to our little lab rodents.

Colossal began with identifying key genetic differences between the woolly mammoth and its closest living relative, the Asian elephant. Scientists focused on genes responsible for traits like thick fur, fat storage, and cold tolerance. Using CRISPR gene-editing technology, researchers modified mouse stem cells to express “mammoth” versions of these traits.

Once the gene-edited stem cells were developed, they were used to create embryos, which were then implanted into surrogate mice. As the embryos in about a dozen surrogate female mice developed, scientists closely monitored the expression of the introduced genes. Finally, the successful birth of the woolly mice confirmed that the introduced genes functioned as intended: all 38 mouse pups were successfully modified to have long, golden hair and faster lipid metabolism.

For Colossal, this achievement was a crucial proof of concept for their larger goal: “bringing back” the woolly mammoth by editing an elephant’s stem cell to achieve the same results. So the next steps involve applying these techniques to elephant cells, with the ultimate goal of creating a mammoth-elephant hybrid capable of thriving in Arctic environments.

“This is really validation that what we have in mind for our longer-term de-extinction project is really going to work,” Beth Shapiro, chief scientific officer at Colossal, told NPR.

Also something to note: as of writing this article (March 13), the paper published by Colossal hasn’t been peer-reviewed yet, so it should be taken with a grain of salt. If anyone’s interested in reading the full paper, here’s the link. I have to say, my favorite woolly mouse is probably Mouse 517. These guys are just so awfully cute.