Cells taken from mice frozen 16 years ago have grown into healthy clones, raising the possibility of reproducing long-dead animals and even resurrecting extinct species.

The feat, accomplished in the laboratory of Japanese geneticist Teruhiko Wakayama, represents a large step forward in animal cloning. Earlier clones have required tissues taken from living animals or carefully preserved cells, rather than an entire frozen animal.

Scientists thought that freezing — of the sort experienced by Wakayama's mice and, for example, a woolly mammoth locked in the Siberian tundra — would damage cells beyond repair. But Wakayama's team salvaged intact nuclei from the neurons of their mice. These were inserted into living mice eggs, forming an embryo that developed until embryonic stem cells could be harvested. The stem cells were then used to make healthy mouse pups.

The previous record for cloning frozen cells, set this summer by Chinese biologist Jinsong Li, was 350 days.

The new technique, wrote Wakayama's team in a paper published yesterday in the Proceedings of the National Academy of Sciences, might someday be used to clone "extinct animals frozen in permafrost, or specimens collected opportunistically from endangered species in the field without access to sophisticated laboratory facilities."