Japanese scientists have cloned healthy mice from bodies kept in deep freeze for 16 years. Skip related content The breakthrough increases the possibility of "resurrecting" extinct animals such as mammoths from their frozen remains. Until now Dolly the Sheep-style cloning has only been achieved using live donor cells, from which DNA is transferred to recipient eggs. Cloning from thawed frozen cells was thought to be difficult, if not impossible, because their DNA would be damaged by ice crystals. This presented a major obstacle to hopes of raising mammoths and other extinct animals preserved in ice from the dead. But a team of scientists from the Centre for Developmental Biology in Kobe, Japan, has now overcome the problem by successfully producing mouse clones from mice frozen at minus 20C for up to 16 years. After thawing out the dead mice, the researchers collected nuclei from cells in their brain tissue. These were injected into empty eggs whose own DNA had been removed, to generate cloned embryos. Stem cells taken from the embryos were then used in a second round of cloning. Their genetic material was inserted into denucleated eggs, to produce embryos that grew into four mouse clones. A further nine "chimeric" mouse clones were created by mixing the cells of different embryos.