De-extinction is the science of bringing back extinct species. But it’s very challenging. Scientists are testing out lots of different ideas. One promising idea is to use genetic engineering to piece together the DNA of an extinct species. We analyzed old, fragmented DNA of the Christmas Island rat, which went extinct about 100 years ago. For an extinct species, the only option is to use the genome of a similar, living species as a reference. By comparing DNA fragments to the reference, you can put the pieces in the right order. We used brown rats as a reference. Unfortunately, we found that some of the genes of the extinct species can’t be recovered. This means that an animal brought back by genetic engineering wouldn’t be an exact copy of the extinct species.