TOKYO -- A research team from the Institute of Physical and Chemical Science (Riken) has succeeded in cloning a mouse using tissue from a mouse that was frozen for 16 years.

The newborn mouse is the first animal in the world to be cloned from frozen animal tissue.

The team's work could potentially lead to the cloning of extinct animals, such as mammoths, but likely will also prove controversial in terms of bioethics.

The research data appeared in yesterday's issue of the journal Proceedings of the National Academy of Sciences.

A cloned animal is created by injecting the nucleus of a cell of the animal to be cloned into an ovum from which the nucleus has been removed, and then transferring the ovum to the womb of a surrogate animal.

Previously, it had not been possible to clone animals from frozen tissue.

But the research team developed a technology to extract the nucleus of a cell by grinding it softly in a special culture. Nuclei from brain and blood cells were taken from a mouse that had been frozen for 16 years.

The nuclei were then inserted into an ovum taken from a healthy mouse. Four mice were born as a result of growing cells derived from embryonic stem cells grown from the ovum and implanting the nuclei of these cells into the eggs of a surrogate mouse.

"Cloning a human is much more difficult than cloning a mouse, and is currently impossible using today's technologies," team leader Teruhiko Wakayama said.

Kinki University professor Akira Iritani, a proponent of a project to revive frozen mammoths, said: "It's believed about 10,000 mammoths are still buried in permafrost in Siberia. [This] might act as a fillip to the mammoth-revival project."